drm: Include drm_connector.h from drm_panel.h

drm_panel.h wants to reference enum drm_panel_orientation which is defined
in drm_connector.h (despite the name).
Include drm_connector.h in drm_panel.h to avoid the rare situation where
drm_panel.h is used with drm_connector.h

https://github.com/raspberrypi/linux/issues/4919



Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>