drm/pci: define drm_pcie_get_speed_cap_mask() only when CONFIG_PCI=y

Move drm_pcie_get_speed_cap_mask() under #ifdef CONFIG_PCI because it
it used only for PCI devices (evergreen, r600, r770), and it uses
PCI interfaces that only exist when CONFIG_PCI=y.

Previously, we tried to compile drm_pcie_get_speed_cap_mask() even when
CONFIG_PCI=n, which fails.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>