drm/amdgpu: add missing lock to amdgpu_ttm_access_memory_sdma

drm/amdgpu: add missing lock to amdgpu_ttm_access_memory_sdma

Users of ttm entities need to hold the gtt_window_lock before using them
to guarantee proper ordering of jobs.

Cc: stable@vger.kernel.org
Fixes: cb5cc4f573e1 ("drm/amdgpu: improve debug VRAM access performance using sdma")
Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>