drm/armada: clean up armada_drm_plane_work_run()

Make armada_drm_plane_work_run() take the drm_plane pointer rather than
our private pointer.  This allows us to localise the conversion between
these two pointers inside armada_drm_plane_work_run(), rather than at
every call site.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>