gma500: udelay(20000) it too long again

so replace it with mdelay(20).

Fixes build error:

  ERROR: "__bad_udelay" [drivers/staging/gma500/psb_gfx.ko] undefined!
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/staging/gma500/mrst_hdmi.c

@@ -129,7 +129,7 @@ static void wait_for_vblank(struct drm_device *dev)
 {
 	/* FIXME: Can we do this as a sleep ? */
 	/* Wait for 20ms, i.e. one cycle at 50hz. */
-	udelay(20000);
+	mdelay(20);
 }
 
 static void scu_busy_loop(void *scu_base)