gma500: remove unused drm_psb_no_fb

commit f9f23a77(gma500: remove no_fb bits)
remove all the drm_psb_no_fb relations code in gma500 except this line code,
so remove it also.
Signed-off-by: Wang YanQing <udknight@gmail.com>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>

drivers/gpu/drm/gma500/psb_drv.h

@@ -876,7 +876,6 @@ extern const struct psb_ops cdv_chip_ops;
 #define PSB_D_MSVDX   (1 << 9)
 #define PSB_D_TOPAZ   (1 << 10)
 
-extern int drm_psb_no_fb;
 extern int drm_idle_check_interval;
 
 /*