drm/amdgpu: reserve fence slot to update page table
Forgot to reserve a fence slot to use sdma to update page table, cause
below kernel BUG backtrace to handle vm retry fault while application is
exiting.
Signed-off-by: Philip Yang <Philip.Yang@amd.com> Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com> Reviewed-by: Christian König <christian.koenig@amd.com> Signed-off-by: Alex Deucher <alexander.deucher@amd.com>