drm/amdgpu: ignore changes of buffer function status because of GPU resets

When we reset the GPU we also disable/enable the SDMA, but we don't want
to change TTM idea of the VRAM size in the middle of that.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Chunming zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>