i965/fs: Avoid unnecessary recompiles due to POS bit of proj_attrib_mask.

Previous to this patch, when using fixed function fragment shading,
bit VARYING_BIT_POS of brw_wm_prog_key::proj_attrib_mask was being set
differently during precompiles and normal usage.  During precompiles
it was being set only if the fragment shader reads from window
position (which it never does), so it was always being set to 0.
During normal usage it was being set if the vertex shader writes to
all 4 components of gl_Position (which it usually does), so it was
usually being set to 1.  As a result, we were almost always doing an
extra recompile for the fixed function fragment shader.

The recompile was totally unnecessary, though, because
brw_wm_prog_key::proj_attrib_mask is only consulted for
fs_visitor::emit_general_interpolation(), which isn't used for
VARYING_SLOT_POS.

This patch avoids the unnecessary recompile by always setting bit
VARYING_BIT_POS of brw_wm_prog_key::proj_attrib_mask to 1.
!UPSTREAM: Hacky fix for Gen5 depthstencil-render-miplevels stencil-only test.  TODO: think through, test thoroughly, and document.
squash! i965: Avoid unnecessary copy when depthstencil workaround invoked by clear.

v3: Use correct bitfields for clear_mask.  When invalidating, make
sure we still reassociate the image to the new miptree.