drm: sun4i: do cleanup if RGB output init fails

sun4i_rgb_init() can fail, which results in TCON failing to bind.
In this case we need to do cleanup, specificly unregistering the
dotclock, which is regmap based, and the regmap is registered as
part of the sun4i_tcon_bind().

Failing to do so results in a NULL pointer reference when the CCF
tries to turn off unused clocks.

Fixes: 29e57fab ("drm: sun4i: Add RGB output")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>