drm/amdgpu: fix rb setting for CZ

Always set num_rbs to 2 for CZ.  The 1 RB parts are often harvest
configs.  The will get sorted out in mesa when we program
PA_SC_RASTER_CONFIG[_1].

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>