drm/omap: Allocate drm_device earlier and unref it as last step

If we allocate the drm_device earlier we can just return the error code
without the need to use goto.
Do the unref of the drm_device as a last step when cleaning up. This will
make the drm_device available longer for us and makes sure that we only
free up the memory when all other cleanups have been already done.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>