drm/sun4i: Make sun4i_crtc_init return ERR_PTR style error codes

sun4i_crtc_init can fail for a number of reasons. Instead of returning
a NULL pointer when it fails, pass back the encountered error using
ERR_PTR.

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