drm/sun4i: mixer: Simplify the get_id logic

Using the new helpers introduced since we wrote that code, we can simplify
the code to retrieve the mixer ID significantly.

The new code will also allow us to deal nicely with endpoints that don't
have a reg property, as expected in the case where there's a single
endpoint for a given port.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Link: https://patchwork.freedesktop.org/patch/msgid/3da40505e18a981c5ad626127e14ff594a826ef5.1552594551.git-series.maxime.ripard@bootlin.com