drm/panel: Add and initialise an orientation field to drm_panel from OF

"rotation" is listed as a standard property of panels in panel-common.yaml,
therefore it would be logical to process that from within the core
code should a panel driver not implement the get_orientation hook.

Call of_drm_get_panel_orientation from
drm_connector_set_orientation_from_panel to get that information.

This removes the need for any boiler-plate in panel drivers for calling
drm_connector_set_orientation_from_panel or
drm_connector_set_panel_orientation.

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