drm: Add helper to implement legacy dirtyfb

Add an atomic helper to implement dirtyfb support.  This is needed to
support DSI command-mode panels with x11 userspace (ie. when we can't
rely on pageflips to trigger a flush to the panel).

v2: Modified the helper to use plane fb_damage_clips property and
removed plane_state::dirty flag.

v3:
- Use uapi drm_mode_rect.
- Support annotate flags.

v4: Correct kernel doc.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Deepak Rawat <drawat@vmware.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>