drm/sun4i: backend: Use drm_fb_cma_get_gem_addr() to get display memory

Commit 4636ce93 ("drm/fb-cma-helper: Add drm_fb_cma_get_gem_addr()")
adds a new helper, which covers fetching a drm_framebuffer's GEM object
and calculating the buffer address for a given plane.

This patch uses this helper to replace our own open coded version of the
same function.

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/20171014040252.9621-4-wens@csie.org