drm/amdgpu/gfx12: fix num_mec

GC12 only has 1 mec.

Fixes: 52cb80c1 ("drm/amdgpu: Add gfx v12_0 ip block support (v6)")
Reviewed-by: Sunil Khatri <sunil.khatri@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>