drm: mali-dp: Add support for writeback on DP550/DP650

drm: mali-dp: Add support for writeback on DP550/DP650

Mali-DP display processors are able to write the composition result to a
memory buffer via the SE.

Add entry points in the HAL for enabling/disabling this feature, and
implement support for it on DP650 and DP550. DP500 acts differently and
so is omitted from this change.

Changes since v3:
 - Fix missing vsync interrupt for DP550

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