drm/radeon/kms: add module option for pcie gen2

Switching to pcie gen2 causes problems on some
boards.  Add a module option to turn it on/off.

There are gen2 compatability issues with some
motherboards it seems.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=33027

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/radeon/evergreen.c b/drivers/gpu/drm/radeon/evergreen.c
index 7fe8ebd..e201a88 100644
--- a/drivers/gpu/drm/radeon/evergreen.c
+++ b/drivers/gpu/drm/radeon/evergreen.c
@@ -3158,6 +3158,9 @@
 {
 	u32 link_width_cntl, speed_cntl;
 
+	if (radeon_pcie_gen2 == 0)
+		return;
+
 	if (rdev->flags & RADEON_IS_IGP)
 		return;