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 <[e-mail]>
Reviewed-by: Daniel Vetter <[e-mail]>

drivers/gpu/drm/bridge/synopsys/dw-hdmi.c

···
 #include <drm/bridge/dw_hdmi.h>
 #include <drm/drm_atomic_helper.h>
 #include <drm/drm_edid.h>
-#include <drm/drm_encoder_slave.h>
 #include <drm/drm_of.h>
 #include <drm/drm_print.h>
 #include <drm/drm_probe_helper.h>

drivers/gpu/drm/imx/dw_hdmi-imx.c

···
 #include <drm/bridge/dw_hdmi.h>
 #include <drm/drm_atomic_helper.h>
 #include <drm/drm_edid.h>
-#include <drm/drm_encoder_slave.h>
+#include <drm/drm_encoder.h>
 #include <drm/drm_of.h>
 
 #include "imx-drm.h"