r600g: remove the "atom" variable from r600_command_buffer

r600g: remove the "atom" variable from r600_command_buffer

r600_command_buffer is not an atom.

The "atoms" have evolved into state slots (or groups of state slots) where
you can bind states. There is a fixed amount of atoms (state slots)
in the context.

The command buffers are nothing like that. They represent states, not state
slots.

We could probably give r600_atom a better name someday.

Reviewed-by: Jerome Glisse <jglisse@redhat.com>