i965/blorp: Rework sRGB override behavior.

i965/blorp: Rework sRGB override behavior.

The previous code for sRGB overrides assumes that the source and
destination formats are equal, other than the color space.  This won't
be feasible when we add support for format conversions.

Here are a few cases, and how the old code handled them:

1.  RGB8 -> SRGB8, MSAA     ==>   SRGB8 -> SRGB8
2.  RGB8 -> SRGB8, single   ==>    RGB8 -> RGB8
3. SRGB8 ->  RGB8, MSAA     ==>    RGB8 -> RGB8
4. SRGB8 ->  RGB8, single   ==>   SRGB8 -> SRGB8

Apparently, preserving the behavior of #1 is important.  When doing a
multisample to single-sample resolve, blending the samples together in
an sRGB correct fashion results in a noticably higher quality image.
It also is necessary to pass Piglit's EXT_framebuffer_multisample
accuracy color tests.

Paul, Eric, Anuj, and I talked about this, and aren't sure that it
matters in the other cases.

This patch preserves the behavior of #1, but otherwise reverts to
doing everything in linear space, changing the behavior of case #4.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>