i965/cnl: Avoid fast-clearing sRGB render buffers

i965/cnl: Avoid fast-clearing sRGB render buffers

Gen10 doesn't automatically decode the clear color of sRGB buffers. To
get correct rendering, avoid fast-clearing such buffers for now.

The driver now passes the following piglit tests:
* spec@arb_framebuffer_srgb@msaa-fast-clear
* spec@ext_texture_srgb@multisample-fast-clear gl_ext_texture_srgb

Suggested-by: Kenneth Graunke <kenneth@whitecape.org>
Suggested-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Nanley Chery <nanley.g.chery@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>