drm/mediatek: Remove redundant calls to drm_connector_register_all()

Up to now, the recommendation was for drivers to call drm_dev_register()
followed by drm_connector_register_all(). Now that
drm_connector_register() is safe against multiple invocations, we can
move drm_connector_register_all() to drm_dev_register() and not suffer
from any backwards compatibility issues with drivers not following the
more rigorous init ordering.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mediatek@lists.infradead.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1466151923-1572-6-git-send-email-chris@chris-wilson.co.uk