drm/mgag200: Black screen fix for G200e rev 4

- Fixed black screen for some resolutions of G200e rev4
- Fixed testm & testn which had predetermined value.

Reported-by: Jan Beulich <jbeulich@suse.com>

Signed-off-by: Mathieu Larouche <mathieu.larouche@matrox.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/mgag200/mgag200_mode.c

@@ -182,7 +182,7 @@ static int mga_g200se_set_plls(struct mga_device *mdev, long clock)
 			}
 		}
 
-		fvv = pllreffreq * testn / testm;
+		fvv = pllreffreq * (n + 1) / (m + 1);
 		fvv = (fvv - 800000) / 50000;
 
 		if (fvv > 15)
@@ -202,6 +202,14 @@ static int mga_g200se_set_plls(struct mga_device *mdev, long clock)
 	WREG_DAC(MGA1064_PIX_PLLC_M, m);
 	WREG_DAC(MGA1064_PIX_PLLC_N, n);
 	WREG_DAC(MGA1064_PIX_PLLC_P, p);
+
+	if (mdev->unique_rev_id >= 0x04) {
+		WREG_DAC(0x1a, 0x09);
+		msleep(20);
+		WREG_DAC(0x1a, 0x01);
+
+	}
+
 	return 0;
 }