drm/amdgpu/gfx11: fix num_mec

GC11 only has 1 mec.

Fixes: 3d879e81 ("drm/amdgpu: add init support for GFX11 (v2)")
Reviewed-by: Sunil Khatri <sunil.khatri@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>