drm/omap: print an error if display enable fails

If the panel's enable fails, omap_encoder silently ignores the failure.
omapdrm should really handle the failure, but unfortunately the whole
encoder enable codepath is expected to always succeed.

So for now, catch the enable failure and print an error.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>