drm/mgag200: fix G200ER pll picking algorithm

The original code was misported from the X driver,

a) an int went to unsigned int, breaking the downward counting testm code
b) the port did the vco/computed clock bits completely wrong.

This fixes an infinite loop on modprobe on some Dell servers with the G200ER
chipset variant.

Found in internal testing.

Cc: stable@vger.kernel.org
Signed-off-by: NDave Airlie <airlied@redhat.com>

drivers/gpu/drm/mgag200/mgag200_mode.c

@@ -468,10 +468,11 @@ static int mga_g200er_set_plls(struct mga_device *mdev, long clock)
 {
 	unsigned int vcomax, vcomin, pllreffreq;
 	unsigned int delta, tmpdelta;
-	unsigned int testr, testn, testm, testo;
+	int testr, testn, testm, testo;
 	unsigned int p, m, n;
-	unsigned int computed;
+	unsigned int computed, vco;
 	int tmp;
+	const unsigned int m_div_val[] = { 1, 2, 4, 8 };
 
 	m = n = p = 0;
 	vcomax = 1488000;
@@ -490,12 +491,13 @@ static int mga_g200er_set_plls(struct mga_device *mdev, long clock)
 				if (delta == 0)
 					break;
 				for (testo = 5; testo < 33; testo++) {
-					computed = pllreffreq * (testn + 1) /
+					vco = pllreffreq * (testn + 1) /
 						(testr + 1);
-					if (computed < vcomin)
+					if (vco < vcomin)
 						continue;
-					if (computed > vcomax)
+					if (vco > vcomax)
 						continue;
+					computed = vco / (m_div_val[testm] * (testo + 1));
 					if (computed > clock)
 						tmpdelta = computed - clock;
 					else