drm/i915: clean up udelay usage

udelay() of 20 milliseconds really ought to just use mdelay(), that avoids
the various wrap scenarios and also is more readable
Signed-off-by: NArjan van de Ven <arjan@linux.intel.com>
Signed-off-by: NEric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/intel_display.c

@@ -636,7 +636,7 @@ void
 intel_wait_for_vblank(struct drm_device *dev)
 {
 	/* Wait for 20ms, i.e. one cycle at 50hz. */
-	udelay(20000);
+	mdelay(20);
 }
 
 static int