drm/amdgpu: prevent get_user_pages recursion

Remember the tasks which are inside get_user_pages()
and ignore MMU callbacks from there.

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