drm/amdgpu: make an improvement on amdgpu_hmm_range_get_pages

Only schedule when hmm_range_fault returns error.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>