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: NDaniel Stone <daniels@collabora.com>
Reported-by: Nkbuild test robot <lkp@intel.com>
Reviewed-by: NThierry 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

drivers/gpu/drm/gma500/mdfld_intel_display.c

@@ -196,7 +196,7 @@ static int mdfld__intel_pipe_set_base(struct drm_crtc *crtc, int x, int y,
 	if (!gma_power_begin(dev, true))
 		return 0;
 
-	start = psbfb->gtt->offset;
+	start = to_gtt_range(fb->obj[0])->offset;
 	offset = y * fb->pitches[0] + x * fb->format->cpp[0];
 
 	REG_WRITE(map->stride, fb->pitches[0]);