drm/pci: Set all supported speeds in speed cap mask for pre-3.0 devices

For devices that conform to PCIe r3.0 and have a Link Capabilities 2
register, we test and report every bit in the Supported Link Speeds Vector
field.  For a device that supports both 2.5GT/s and 5.0GT/s, we set both
DRM_PCIE_SPEED_25 and DRM_PCIE_SPEED_50 in the returned mask.

For pre-r3.0 devices, the Link Capabilities 0010b encoding
(PCI_EXP_LNKCAP_SLS_5_0GB) means that both 5.0GT/s and 2.5GT/s are
supported, so set both DRM_PCIE_SPEED_25 and DRM_PCIE_SPEED_50 in this
case as well.

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