drm/vc4: Call drm_dev_register() after all setup is done

drm_dev_register() initializes internal clients like bootsplash as the
last thing it does, so all setup needs to be done at this point.

Fix by calling vc4_kms_load() before registering.
Also check the error code returned from that function.

Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190326175546.18126-17-noralf@tronnes.org