drm: mali-dp: Add RGB writeback formats for DP550/DP650

Add a layer bit for the SE memory-write, and add it to the pixel format
matrix for DP550/DP650.

Signed-off-by: Brian Starkey <brian.starkey@arm.com>
[rebased and fixed conflicts]
Signed-off-by: Mihail Atanassov <mihail.atanassov@arm.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>