drm/sun4i: rgb: Store the bridge pointer

We'll need the bridge pointer, if any, in the mode_valid callback in
addition to the init function. Store the pointer to the bridge in the
rgb private structure.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Tested-by: Vasily Khoruzhick <anarsoul@gmail.com> # tested on pinebook
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/2f16d70fb09613b3a030fd6a016343047d519d43.1551191081.git-series.maxime.ripard@bootlin.com