drm/amdgpu: check before checking pci bridge registers

Make sure we are not the root device before attempting to
read the pcie bridge registers to check the pcie gen speeed.

Fixes a crash when the device is passed through to a VM.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
diff --git a/drivers/gpu/drm/amd/amdgpu/vi.c b/drivers/gpu/drm/amd/amdgpu/vi.c
index b55ceb1..0bac8702 100644
--- a/drivers/gpu/drm/amd/amdgpu/vi.c
+++ b/drivers/gpu/drm/amd/amdgpu/vi.c
@@ -1005,6 +1005,9 @@
 	u32 mask;
 	int ret;
 
+	if (pci_is_root_bus(adev->pdev->bus))
+		return;
+
 	if (amdgpu_pcie_gen2 == 0)
 		return;