drm/amdgpu: Pin MMIO/DOORBELL BO's in GTT domain

MMIO/DOORBELL BOs encode control data and should be pinned in GTT
domain before enabling PCIe connected peer devices in accessing it

Signed-off-by: Ramesh Errabolu <Ramesh.Errabolu@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>