drm/tinydrm: Use drm_gem_framebuffer_helper

Use drm_gem_framebuffer_helper directly instead of the cma
library wrappers.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/1506255985-61113-2-git-send-email-noralf@tronnes.org