drm/sun4i: Add backend and tcon pointers to sun4i_crtc

sun4i_crtc controls the backend and tcon hardware blocks of the display
pipeline. Instead of doing so through the master drm structure, leave
pointers to the corresponding backend and tcon in itself.

Also drop the drm_device pointer, since it is no longer needed.

The next step forward would be to pass the pointers in through
sun4i_crtc_init as parameters. This would make it easier to support
multiple display pipelines layer on.

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