i965/gen7+: Implement fast color clear operation in BLORP.

Since we defer allocation of the MCS miptree until the time of the
fast clear operation, this patch also implements creation of the MCS
miptree.

In addition, this patch adds the field
intel_mipmap_tree::fast_clear_color_value, which holds the most recent
fast color clear value, if any. We use it to set the SURFACE_STATE's
clear color for render targets.

v2: Flag BRW_NEW_SURFACES when allocating the MCS miptree.  Generate a
perf_debug message if clearing to a color that isn't compatible with
fast color clear.  Fix "control reaches end of non-void function"
build warning.
i965/blorp: Expand clear class hierarchy to prepare for RT resolves.

The fragment shaders that to do color clears will be re-used to
perform so-called "render target resolves" (the resolves associated
with fast color clears).  To prepare for that, this patch expands the
class hierarchy for blorp params by adding
brw_blorp_const_color_params (which will be used for all blorp
operations where the fragment shader outputs a constant color).

Some other data structures and functions were also renamed to use
"const_color" nomenclature where appropriate.
i965/blorp: Write blorp code to do render target resolves.

This patch implements the "render target resolve" blorp operation.
This will be needed when a buffer that has experienced a fast color
clear is later used for a purpose other than as a render target
(texturing, glReadPixels, or swapped to the screen).  It resolves any
remaining deferred clear operation that was not taken care of during
normal rendering.

Fortunately not much work is necessary; all we need to do is scale
down the size of the rectangle primitive being emitted, run the
fragment shader with the "Render Target Resolve Enable" bit set, and
ensure that the fragment shader writes to the render target using the
"replicated color" message.  We already have a fragment shader that
does that (the shader that we use for fast color clears), so for
simplicity we re-use it.
i965/gen7+: Ensure that front/back buffers are fast-clear resolved.

We already had code in intel_downsample_for_dri2_flush() for
downsampling front and back buffers when multisampling was in use.
This patch extends that function to perform fast color clear resolves
when necessary.

To account for the additional functionality, the function is renamed
to simply intel_resolve_for_dri2_flush().
i965/gen7+: Resolve color buffers when necessary.

Resolve color buffers that have been fast-color cleared:
    1. before texturing from the buffer
    2. before using the buffer as the source in a blorp blit
    3. before mapping the buffer's miptree
    4. before accessing the buffer using the hardware blitter

Cases 1 and 2 happen in the functions brw_predraw_resolve_buffers()
and brw_blorp_blit_miptrees(), respectively.  Cases 3 and 4 are
handled by the intel_miptree_get_region() function.

In order to make sure that intel_miptree_get_region() doesn't try to
resolve a buffer during emission of state commands, this patch also
adds a new boolean, brw->state_emission_in_progress, which is set to
true during brw_upload_state() and brw_blorp_exec().
i965/gen7+: Disable fast color clears on shared regions.

In certain circumstances the memory region underlying a miptree is
shared with other miptrees, or with other code outside Mesa's control.
This happens, for instance, when an extension like GL_OES_EGL_image or
GLX_EXT_texture_from_pixmap extension is used to associate a miptree
with an image existing outside of Mesa.

When this happens, we need to disable fast color clears on the miptree
in question, since there's no good synchronization mechanism to ensure
that deferred clear writes get performed by the time the buffer is
examined from the other miptree, or from outside of Mesa.

Fortunately, this should not be a performance hit for most
applications, since most applications that use these extensions use
them for importing textures into Mesa, rather than for exporting
rendered images out of Mesa.  So most of the time the miptrees
involved will never experience a clear.
i965/gen7: Enable support for fast color clears.

This patch adds code to place mcs_state into INTEL_MCS_STATE_RESOLVED
for miptrees that are capable of supporting fast color clears.  This
will have no effect on buffers that don't undergo a fast color clear;
however, for buffers that do undergo a fast color clear, an MCS
miptree will be allocated (at the time of the first fast clear), and
will be used thereafter.