drm: mga: set dev_priv_size

fd.o bug 1746
Signed-off-by: Dave Airlie <airlied@linux.ie>

drivers/char/drm/mga_drv.c

@@ -47,6 +47,7 @@ static struct drm_driver driver = {
 	    DRIVER_USE_AGP | DRIVER_USE_MTRR | DRIVER_PCI_DMA |
 	    DRIVER_HAVE_DMA | DRIVER_HAVE_IRQ | DRIVER_IRQ_SHARED |
 	    DRIVER_IRQ_VBL,
+	.dev_priv_size = sizeof(drm_mga_buf_priv_t),
 	.load = mga_driver_load,
 	.unload = mga_driver_unload,
 	.lastclose = mga_driver_lastclose,