drm: mali-dp: Add writeback connector

Mali-DP has a memory writeback engine which can be used to write the
composition result to a memory buffer. Expose this functionality as a
DRM writeback connector on supported hardware.

Changes since v1:
 Daniel Vetter:
 - Don't require a modeset when writeback routing changes
 - Make writeback connector always disconnected

Changes since v2:
 - Rebase onto new drm_writeback_connector
 - Add reset callback, allocating subclassed state
 Daniel Vetter:
 - Squash out-fence support into this commit
 Gustavo Padovan:
 - Don't signal fence directly from driver (and drop malidp_mw_job)

Changes since v3:
 - Modifications to fit with Mali-DP commit tail changes

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>