PCI Speed Cap Fixes for ppc64

[lucaskt at linux.vnet.ibm.com]

This patch series at first moves get_speed_cap_mask from DRM to PCI, fixes all
radeon references (only driver that uses it) and then implements a architecture
specific implementation for ppc64 for it.

This is good because the drm_pcie_get_speed_cap_mask function is more
architecture goo than something drm-specific, and also because this way
architectures that have different means of obtaining these values can do it
their way.