For texturing and draw calls, HW expects the clear color to be in two
different color spaces after sRGB fast-clears - sRGB in the former and
linear in the latter. Up until now, iris has stored the clear color in
the sRGB color space. Limit the allowable clear colors for sRGB
fast-clears to 0/1 so that both color space requirements are satisfied.

Makes iris pass the sRGB -> sRGB subtest of the fcc-write-after-clear
piglit test on gen9+.

v2:
* Drop iris_context::blend_enables. (Ken)
* Drop some more resolve-related blend-state-tracking code.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <mesa/mesa!4972>