drm/amdgpu: Force order between a read and write to the same address

Setting register to force ordering to prevent read/write or write/read
hazards for un-cached modes.

Signed-off-by: Alex Sierra <alex.sierra@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>