drm/sun4i: Pass pointers for associated backend and tcon into crtc init

sun4i_crtc controls the backend and tcon hardware blocks of the display
pipeline.

Pass pointers to the underlying devices into the crtc init function,
instead of trying to fetch them from the drm_device structure. This
avoids the headache of trying to figure out which devices the crtc
is actually associated with.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>