drm/gma500: Fix Medfield for drm_framebuffer move

Commit bc61c975

 ("drm/gma500: Move GEM BO to drm_framebuffer") moved
the gtt_range structure, from being in psb_framebuffer and embedding the
GEM object, to being placed in the drm_framebuffer with the gtt_range
being derived from the GEM object.

The conversion missed out the Medfield subdriver, which was not being
built in the default drm-misc config. Do the trivial fixup here.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reported-by: kbuild test robot <lkp@intel.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Fixes: bc61c975 ("drm/gma500: Move GEM BO to drm_framebuffer")
Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180521142449.20800-1-daniels@collabora.com