drm/amdgpu: use pcie_bandwidth_available rather than open coding it

It does the same thing we were doing already.  I though it needed
work for gen3/4 speeds, but that seems to be covered already.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Acked-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>