i965/gen7+: Disentangle MSAA layout from fast clear state.

This patch renames the enum that's used to keep track of fast clear
state from "mcs_state" to "fast_clear_state", and it removes the enum
value INTEL_MCS_STATE_MSAA (which previously meant, "this is an MSAA
buffer, so we're not keeping track of fast clear state").  The only
real purpose that enum value was serving was to prevent us from trying
to do fast clear resolves on MSAA buffers, and it's just as easy to
prevent that by checking the buffer's msaa_layout.

This paves the way for implementing fast clears of MSAA buffers.
!UPSTREAM: allow multisample blorp clears
Non-helpful hacks
HACK: try UMS layout
HACK: make the rendering pipeline multisample state match the destination multisample state
i965/blorp: Get rid of redundant num_samples blorp param.

Previously, brw_blorp_params contained two fields for determining
sample count: num_samples (which determined the multisample
configuration of the rendering pipeline) and dst.num_samples (which
determined the multisample configuration of the render target
surface).  This was redundant, since both fields had to be set to the
same value to avoid rendering errors.

This patch eliminates num_samples to avoid future confusion.