intel/blorp: Handle 3-component formats in clears

This fixes a nasty hang in Batman: Arkham City which apparently calls
vkCmdClearColorImage on a linear RGB image.

cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
(cherry picked from commit daa78f30)

Conflicts resolved by Dylan

Conflicts:
	src/intel/blorp/blorp_blit.c