drm/msm: Move GEM BOs to drm_framebuffer

Since drm_framebuffer can now store GEM objects directly, place them
there rather than in our own subclass. As this makes the framebuffer
create_handle function the same as the GEM framebuffer helper, we
can reuse that.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: linux-arm-msm@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20180330141138.28987-21-daniels@collabora.com