r600g: turn init_config into a command buffer for starting a CS

r600g: turn init_config into a command buffer for starting a CS

This is the first pure command buffer. It contains CS initialization
packets and emits invariant state (i.e. the registers which never or rarely
change).

The affected registers are removed from *_hw_context.c, so that both ways
of emitting commands can co-exist.

v2: emit context_control in cayman's start_cs too