drm/armada: switch legacy modeset to atomic modeset

Switch the legacy set_config() method to use the atomic modeset
helper, which allows us to get rid of the legacy dpms, prepare,
commit, mode_set, mode_set_base and disable helper methods.

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

drivers/gpu/drm/armada/armada_crtc.h

@@ -83,7 +83,6 @@ struct armada_crtc {
 	uint32_t		cursor_w;
 	uint32_t		cursor_h;
 
-	int			dpms;
 	uint32_t		cfg_dumb_ctrl;
 	uint32_t		spu_iopad_ctrl;