drm/armada: clean up armada_drm_crtc_plane_disable()

Merge armada_drm_primary_disable() into armada_drm_crtc_plane_disable()
and rename to armada_drm_plane_disable().  Use this to simplify
armada_ovl_plane_disable().

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