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
diff --git a/Documentation/gpu/drm-kms-helpers.rst b/Documentation/gpu/drm-kms-helpers.rst
index 7c5e254..13dd237 100644
--- a/Documentation/gpu/drm-kms-helpers.rst
+++ b/Documentation/gpu/drm-kms-helpers.rst
@@ -296,3 +296,12 @@
 
 .. kernel-doc:: drivers/gpu/drm/drm_modeset_helper.c
    :export:
+
+Framebuffer GEM Helper Reference
+================================
+
+.. kernel-doc:: drivers/gpu/drm/drm_gem_framebuffer_helper.c
+   :doc: overview
+
+.. kernel-doc:: drivers/gpu/drm/drm_gem_framebuffer_helper.c
+   :export: