i965/meta-fast-clear: Disable GL_FRAMEBUFFER_SRGB during clear

i965/meta-fast-clear: Disable GL_FRAMEBUFFER_SRGB during clear

Adds MESA_META_FRAMEBUFFER_SRGB to the meta save state so that
GL_FRAMEBUFFER_SRGB will be disabled when performing the fast clear.
That way the render surface state will be programmed with the linear
equivalent format during the clear. This is important for Gen9 because
the SRGB formats are not marked as losslessly compressible so in
theory they aren't support for fast clears. It shouldn't make any
difference whether GL_FRAMEBUFFER_SRGB is enabled for the fast clear
operation because the color is not actually written to the framebuffer
so there is no chance for the hardware to apply the SRGB conversion on
it anyway.

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