drm: Add GEM backed framebuffer library

This library provides helpers for drivers that don't subclass
drm_framebuffer and are backed by drm_gem_object. The code is
taken from drm_fb_cma_helper.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/1502631125-13557-2-git-send-email-noralf@tronnes.org