drm: Don't include drm/drm_encoder_slave.h when not needed

The dw-hdmi, kirin and imx drivers include the drm/drm_encoder_slave.h
header but don't use the encoder slave API. Remove it or replace it with
drm/drm_encoder.h as needed.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>