drm/amdgpu/vi: remove duplicate CG flags

GFX_MGLS was added twice.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>