drm: add drmm_encoder_alloc()

Add an alternative to drm_encoder_init() that allocates and initializes
an encoder and registers drm_encoder_cleanup() with
drmm_add_action_or_reset().

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>