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/r600.c b/drivers/gpu/drm/radeon/r600.c
index 6b50716..9fe86253 100644
--- a/drivers/gpu/drm/radeon/r600.c
+++ b/drivers/gpu/drm/radeon/r600.c
@@ -3658,6 +3658,9 @@
 	u32 link_width_cntl, lanes, speed_cntl, training_cntl, tmp;
 	u16 link_cntl2;
 
+	if (radeon_pcie_gen2 == 0)
+		return;
+
 	if (rdev->flags & RADEON_IS_IGP)
 		return;