i965: Add and use a getter for the clear color

i965: Add and use a getter for the clear color

It returns both the inline clear color and a clear address which points
to the indirect clear color buffer (or NULL if unused/non-existent).
This getter allows CNL to sample from fast-cleared sRGB textures
correctly by doing the needed sRGB-decode on the clear color (inline)
and making the indirect clear color buffer unused.

v2 (Rafael):
* Have a more detailed commit message.
* Add a comment on the sRGB conversion process.

Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>