hwcomposer: Use RGB_BORDERFILL only on MDP 4.2 and higher.

MDP 4.0 does not support RGB_BORDERFILL and causes the display to fail
initializing.

Patch Set 2: Actually set MDP_RGB format if not BORDERFILL.

Change-Id: Ib2007221c4dc78fd17ced97c0a0d28896c877b80