amd/amdgpu: Inherit coherence flags base on original BO flags

For SG BO to DMA-map userptrs on other GPUs, the SG BO
need inherit MTYPEs in PTEs from original BO.

If we set the flags, the device can be coherent with
the CPUs and other GPUs.

v2:
  1. Drop unnecessary flags check
  2. Remove local variable align

Signed-off-by: Shane Xiao <shane.xiao@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>