drm/amdgpu: Prepare for hmm_range_register API change (v2)

An upcoming change in the hmm_range_register API requires passing in
a pointer to an hmm_mirror instead of mm_struct. To access the
hmm_mirror we need pass bo instead of ttm to amdgpu_ttm_tt_get_user_pages
because mirror is part of amdgpu_mn structure, which is accessible from bo.

v2: fix building without CONFIG_HMM_MIRROR (Arnd)

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