drm/radeon: use pcie functions for link width

This is the last user of drm_pcie_get_speed_cap_mask.  Use the pci
version so we can drop drm_pcie_get_speed_cap_mask.

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