gralloc: Fix RGB888 marked as VIDEO

RGB888 layers were marked as VIDEO and treated as YUV along the path.
If RGB888 is ever required to be a video format we need to make sure
to have other differentiation mechanisms between a UI RGB888 and a
Video RGB888 if any special handling is necessary.

Change-Id: I94c74ac10d589e20dd600db0d1e32645613b1b21