radeonsi: disallow adding BOs into si_pm4_state except 1 shader BO per state

radeonsi: disallow adding BOs into si_pm4_state except 1 shader BO per state

The si_shader pointer is already there, so use it and remove the array
of BOs.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5603>