drm/sun4i: Use drm_gem_fb_create()

drm_fb_cma_create() is just a wrapper around drm_gem_fb_create() now,
so use the function directly.

Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/1506255985-61113-9-git-send-email-noralf@tronnes.org