drm/mgag200: Add missing write to index before accessing data register

This is a bug fix for some versions of g200se cards while doing
mode-setting.

Signed-off-by: Christopher Harvey <charvey@matrox.com>
Tested-by: Julia Lemire <jlemire@matrox.com>
Acked-by: Julia Lemire <jlemire@matrox.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@gmail.com>

drivers/gpu/drm/mgag200/mgag200_mode.c

@@ -1034,13 +1034,14 @@ static int mga_crtc_mode_set(struct drm_crtc *crtc,
 			else
 				hi_pri_lvl = 5;
 
-			WREG8(0x1fde, 0x06);
-			WREG8(0x1fdf, hi_pri_lvl);
+			WREG8(MGAREG_CRTCEXT_INDEX, 0x06);
+			WREG8(MGAREG_CRTCEXT_DATA, hi_pri_lvl);
 		} else {
+			WREG8(MGAREG_CRTCEXT_INDEX, 0x06);
 			if (mdev->reg_1e24 >= 0x01)
-				WREG8(0x1fdf, 0x03);
+				WREG8(MGAREG_CRTCEXT_DATA, 0x03);
 			else
-				WREG8(0x1fdf, 0x04);
+				WREG8(MGAREG_CRTCEXT_DATA, 0x04);
 		}
 	}
 	return 0;