i965: Pass flags to brw_bo_map_*

i965: Pass flags to brw_bo_map_*

brw_bo_map_cpu() took a write_enable arg, but it wasn't always clear
whether we were also planning to read from the buffer. I kept everything
semantically identical by passing only MAP_READ or MAP_READ | MAP_WRITE
depending on the write_enable argument.

The other flags are not used yet, but MAP_ASYNC for instance, will be
used in a later patch to remove the need for a separate
brw_bo_map_unsynchronized() function.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>