i965: Move flushing out of write_reg and into the callers.

i965: Move flushing out of write_reg and into the callers.

The current callers just want to write a single register, so combining
the register read with a pipeline flush made sense.  However, in the
future we'll want to do multiple register reads back to back, and we'll
only want to flush once.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>