i965/meta_util: Re-enable sRGB-encoded fast-clears on CNL

i965/meta_util: Re-enable sRGB-encoded fast-clears on CNL

The paths which sample with the clear color are now using a getter which
performs the sRGB decode needed to enable this fast clear.

This path can be exercised by fast-clearing a texture, then performing
an operation which requires sRGB decoding. Test coverage for this
feature is provided with the following tests:

* Shader texture calls:
  - spec@ext_texture_srgb@tex-srgb

* Shader texelfetch calls:
  - spec@arb_framebuffer_srgb@fbo-fast-clear
  - spec@arb_framebuffer_srgb@msaa-fast-clear

* Blending:
  - spec@arb_framebuffer_srgb@arb_framebuffer_srgb-fast-clear-blend

* Blitting:
  - spec@arb_framebuffer_srgb@blit texture srgb msaa enabled clear

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>