i965/gen9: Allow fast clear for MSRT formats matching render

i965/gen9: Allow fast clear for MSRT formats matching render

Previously fast clear was disallowed on Gen9 for MSRTs with the claim
that some formats don't work but we didn't understand why. On further
investigation it seems the formats that don't work are the ones where
the render surface format is being overriden to a different format
than the one used for texturing. The one used for texturing is not
actually a renderable format. It arguably makes sense that the sampler
hardware doesn't handle the fast color correctly in these cases
because it shouldn't be possible to end up with a fast cleared surface
that is non-renderable.

This patch changes the limitation to prevent fast clear for surfaces
where the format for rendering is overriden.

Reviewed-by: Ben Widawsky <benjamin.widawsky@intel.com>