drm/amdkfd: Support two MMHUBs when setting up page table base in KFD

2 mmhubs on arcturus.

Signed-off-by: Yong Zhao <Yong.Zhao@amd.com>
Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>