drm/sun4i: backend: Create regmap after access is possible

The backend has various clocks and reset controls that need to be
enabled and deasserted before register access is possible.

Move the creation of the regmap to after the clocks and reset controls
have been configured where it makes more sense.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171014040252.9621-3-wens@csie.org