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: c7cbed56

 ("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