drm/sun4i: Drop primary layer pointer from sun4i_drv

The current layer init code keeps a pointer to the primary plane layer
in sun4i_drv. When we eventually support multiple display pipelines,
this would force us to keep track of primary planes for all crtcs. And
these pointers only get used at bind time.

Instead, have the crtc init code iterate through the returned layers
to find the primary and cursor layers. And drop the pointer from the
sun4i_drv structure.

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