drm/fb-cma-helper: Use drm_gem_framebuffer_helper

Use the new drm_gem_framebuffer_helper who's code was copied
from this helper.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/1502631125-13557-3-git-send-email-noralf@tronnes.org