gma500: do a pass over the FIXME tags

drivers/gpu/drm/gma500/cdv_device.c

drivers/gpu/drm/gma500/gem.c

drivers/gpu/drm/gma500/gtt.c

drivers/gpu/drm/gma500/intel_opregion.c

drivers/gpu/drm/gma500/oaktrail_crtc.c

drivers/gpu/drm/gma500/psb_drv.c

drivers/gpu/drm/gma500/psb_intel_lvds.c

drivers/gpu/drm/gma500/psb_lid.c