i965/gen9: Allow fast clears for non-MSRT SRGB buffers

i965/gen9: Allow fast clears for non-MSRT SRGB buffers

SRGB buffers are not marked as losslessly compressible so previously
they would not be used for fast clears. However in practice the
hardware will never actually see that we are using SRGB buffers for
fast clears if we use the linear equivalent format when clearing and
make sure to resolve the buffer as a linear format before sampling
from it.

This is an important use case because by default the window system
framebuffers are created as SRGB so without this fast clears won't be
used there.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>