drm/amd/amdgpu: Prevent null pointer dereference in GPU bandwidth calculation

If the parent is NULL, adev->pdev is used to retrieve the PCIe speed and
width, ensuring that  the function can still determine these
capabilities from the device itself.

Fixes the below:
drivers/gpu/drm/amd/amdgpu/amdgpu_device.c:6193 amdgpu_device_gpu_bandwidth()
	error: we previously assumed 'parent' could be null (see line 6180)

drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
    6170 static void amdgpu_device_gpu_bandwidth(struct amdgpu_device *adev,
    6171                                         enum pci_bus_speed *speed,
    6172                                         enum pcie_link_width *width)
    6173 {
    6174         struct pci_dev *parent = adev->pdev;
    6175
    6176         if (!speed || !width)
    6177                 return;
    6178
    6179         parent = pci_upstream_bridge(parent);
    6180         if (parent && parent->vendor == PCI_VENDOR_ID_ATI) {
                     ^^^^^^
If parent is NULL

    6181                 /* use the upstream/downstream switches internal to dGPU */
    6182                 *speed = pcie_get_speed_cap(parent);
    6183                 *width = pcie_get_width_cap(parent);
    6184                 while ((parent = pci_upstream_bridge(parent))) {
    6185                         if (parent->vendor == PCI_VENDOR_ID_ATI) {
    6186                                 /* use the upstream/downstream switches internal to dGPU */
    6187                                 *speed = pcie_get_speed_cap(parent);
    6188                                 *width = pcie_get_width_cap(parent);
    6189                         }
    6190                 }
    6191         } else {
    6192                 /* use the device itself */
--> 6193                 *speed = pcie_get_speed_cap(parent);
                                                     ^^^^^^ Then we are toasted here.

    6194                 *width = pcie_get_width_cap(parent);
    6195         }
    6196 }

Fixes: 757e8b95 ("drm/amdgpu: cache gpu pcie link width")
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Suggested-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>