drm/gma500: Fix compile warning

Fix the following compile warning:

warning: unused variable ‘psbfb’ [-Wunused-variable]
  struct psb_framebuffer *psbfb = to_psb_fb(fb);

Fixes: c7cbed560ce2 ("drm/gma500: Fix Medfield for drm_framebuffer move")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180625123355.GA16757@embeddedor.com

drivers/gpu/drm/gma500/mdfld_intel_display.c

@@ -167,7 +167,6 @@ static int mdfld__intel_pipe_set_base(struct drm_crtc *crtc, int x, int y,
 	struct drm_psb_private *dev_priv = dev->dev_private;
 	struct drm_framebuffer *fb = crtc->primary->fb;
 	struct gma_crtc *gma_crtc = to_gma_crtc(crtc);
-	struct psb_framebuffer *psbfb = to_psb_fb(fb);
 	int pipe = gma_crtc->pipe;
 	const struct psb_offset *map = &dev_priv->regmap[pipe];
 	unsigned long start, offset;