mesa: Set green, blue channels to zero only for formats with these components

mesa: Set green, blue channels to zero only for formats with these components

This is an optimization which avoids setting pixel transfer operations
when not required. _mesa_ReadPixels falls back to slower path if
transfer operations are set.

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>