drm/amdgpu: Add use_xgmi_p2p module parameter

This parameter controls xGMI p2p communication, which is enabled by
default. However, it can be disabled by setting it to 0. In case xGMI
p2p is disabled in a dGPU, PCIe p2p interface will be used instead.
This parameter is ignored in GPUs that do not support xGMI
p2p configuration.

Signed-off-by: Alex Sierra <alex.sierra@amd.com>
Acked-by: Luben Tuikov <luben.tuikov@amd.com>
Acked-by: Harish Kasiviswanathan <Harish.Kasiviswanathan@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>