mesa: Drop PIPE_CAP_PREFERRED_IR, always producing NIR from mesa/state_tracker

Functional changes from this MR should be the PBO GS path being taken once again (lost in the previous frontend NIR transition), and SVGA ingesting NIR instead of TGSI (it does its own nir_to_tgsi() call anyway). The benefit is fewer untested debug paths and less complexity in st_program.c