drm/amdgpu: drop redundant null-pointer checks in amdgpu_ttm_tt_populate() and amdgpu_ttm_tt_unpopulate()

The varialbe gtt in the function amdgpu_ttm_tt_populate() and
amdgpu_ttm_tt_unpopulate() is guaranteed to be not NULL in the context.
Thus the null-pointer checks are redundant and can be dropped.
Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Tuo Li <islituo@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>