i965: Remove the rest of brw_update_draw_buffer().

i965: Remove the rest of brw_update_draw_buffer().

The last piece of code with an effect was flagging _NEW_BUFFERS.  Only,
that is already flagged from everything that calls this function: Mesa GL
state updates flag it before even calling down into the driver, and the
calls from the DRI2 window system framebuffer update path end up flagging
it as part of the ResizeBuffers() hook.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>