drm/amdgpu: correct MEC number for gfx11 APUs

There is only one MEC on these APUs.

Signed-off-by: Lang Yu <Lang.Yu@amd.com>
Reviewed-by: Aaron Liu <aaron.liu@amd.com>
Reviewed-by: Yifan Zhang <yifan1.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org # 6.1.x