modesetting: Empty damage once dispatch is done

This commit is to fix some regression caused by

commit 995e60a919

where DamageEmpty on damage was missing in several
places.

Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1486>

hw/xfree86/drivers/video/modesetting/driver.c

@@ -782,6 +782,8 @@ dispatch_dirty(ScreenPtr pScreen)
             return;
         }
     }
+
+    DamageEmpty(ms->damage);
 }
 
 static void
@@ -793,6 +795,8 @@ dispatch_dirty_pixmap(ScrnInfoPtr scrn, xf86CrtcPtr crtc, PixmapPtr ppix)
     int fb_id = ppriv->fb_id;
 
     dispatch_dirty_region(scrn, crtc, ppix, damage, fb_id, 0, 0);
+    if (damage)
+        DamageEmpty(damage);
 }
 
 static void