iris: Allocate buffer space for the fast clear color.

iris: Allocate buffer space for the fast clear color.

Also store clear color in the iris_resource.

Always allocate clear color state buffer.

v2:
 - Make clear_color_offset be 64 bits (Ken).
 - Simplify the logic to decide when to memset the aux buffer (Ken).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>