drm/radeon/kms: fix r300 vram width calculations

This was incorrect according to the docs and the UMS driver does
it like this.

Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/r300.c

@@ -506,11 +506,14 @@ void r300_vram_info(struct radeon_device *rdev)
 
 	/* DDR for all card after R300 & IGP */
 	rdev->mc.vram_is_ddr = true;
+
 	tmp = RREG32(RADEON_MEM_CNTL);
-	if (tmp & R300_MEM_NUM_CHANNELS_MASK) {
-		rdev->mc.vram_width = 128;
-	} else {
-		rdev->mc.vram_width = 64;
+	tmp &= R300_MEM_NUM_CHANNELS_MASK;
+	switch (tmp) {
+	case 0: rdev->mc.vram_width = 64; break;
+	case 1: rdev->mc.vram_width = 128; break;
+	case 2: rdev->mc.vram_width = 256; break;
+	default:  rdev->mc.vram_width = 128; break;
 	}
 
 	r100_vram_init_sizes(rdev);