video/fb: Propagate error code from failing to unregister conflicting fb

If we fail to remove a conflicting fb driver, we need to abort the
loading of the second driver to avoid likely kernel panics.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: linux-fbdev@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>