PCI: Export pcie_get_speed_cap and pcie_get_width_cap

So drivers can use them.  This can be used to replace
duplicate code in the drm subsystem.

Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>