radeonsi: add a separate dirty mask for prefetches

radeonsi: add a separate dirty mask for prefetches

so that we don't rely on si_pm4_state_enabled_and_changed, allowing us
to move prefetches after draw calls.

v2: ckear the dirty mask after unbinding shaders

Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de> (v1)
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com> (v1)