anv: Be more careful about fast-clear colors

anv: Be more careful about fast-clear colors

Previously, we just used all the channels regardless of the format.
This is less than ideal because some channels may have undefined values
and this should be ok from the client's perspective.  Even though the
driver should do the correct thing regardless of what is in the
undefined value, it makes things less deterministic.  In particular, the
driver may choose to fast-clear or not based on undefined values.  This
level of nondeterminism is bad.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>