i965/gen9/fast-clear: Handle linear→SRGB conversion

i965/gen9/fast-clear: Handle linear→SRGB conversion

If GL_FRAMEBUFFER_SRGB is enabled when writing to an SRGB-capable
framebuffer then the color will be converted from linear to SRGB
before being written. There is no chance for the hardware to do this
itself because it can't modify the clear color that is programmed in
the surface state so it seems pretty clear that the driver should be
handling this itself.

Note that this wasn't a problem before Gen9 because previously we were
only able to do fast clears to 0 or 1 and those values are the same in
linear and SRGB space.

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