drm/radeon/kms: respect single crtc cards, only create one crtc. (v2)

Also add single crtc for RN50 chips.

changes in v2:
fix vblank init to respect single crtc flag
fix r100 mode bandwidth to respect single crtc flag

Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/radeon/r100.c b/drivers/gpu/drm/radeon/r100.c
index 7bea923..5fe12c0 100644
--- a/drivers/gpu/drm/radeon/r100.c
+++ b/drivers/gpu/drm/radeon/r100.c
@@ -2135,9 +2135,11 @@
 		mode1 = &rdev->mode_info.crtcs[0]->base.mode;
 		pixel_bytes1 = rdev->mode_info.crtcs[0]->base.fb->bits_per_pixel / 8;
 	}
-	if (rdev->mode_info.crtcs[1]->base.enabled) {
-		mode2 = &rdev->mode_info.crtcs[1]->base.mode;
-		pixel_bytes2 = rdev->mode_info.crtcs[1]->base.fb->bits_per_pixel / 8;
+	if (!(rdev->flags & RADEON_SINGLE_CRTC)) {
+		if (rdev->mode_info.crtcs[1]->base.enabled) {
+			mode2 = &rdev->mode_info.crtcs[1]->base.mode;
+			pixel_bytes2 = rdev->mode_info.crtcs[1]->base.fb->bits_per_pixel / 8;
+		}
 	}
 
 	min_mem_eff.full = rfixed_const_8(0);