drm/pci: Use PCI Express Capability accessors

Use PCI Express Capability access functions to simplify this code a bit.
For non-PCIe devices or pre-PCIe 3.0 devices that don't implement the Link
Capabilities 2 register, pcie_capability_read_dword() reads a zero.

Since we're only testing whether the bits we care about are set, there's no
need to mask out the other bits we *don't* care about.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>