drm/amdkfd: Use functions from amdgpu for setting up page table base

Use the functions from amdgpu to avoid directly programming registers in
amdgpu_amdkfd_gfx_v9.c.

Signed-off-by: Yong Zhao <Yong.Zhao@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>