drm/amd/amdgpu: Drop commented out stub function

Drop the function gmc_v6_0_init_compute_vmid() since it wasn't
implemented and commented out.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>