i965: Add and use brw_bo_map()

i965: Add and use brw_bo_map()

We can encapsulate the logic for choosing the mapping type. This will
also help when we add WC mappings.

A few functional changes are made in this patch. On non-LLC, what were
previously WB mappings are now GTT mappings (in the prefilling debug
code in brw_performance_query.c; the shader_time code in brw_program.c;
and in the case of an RW mapping in intel_buffer_objects.c).

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