i965: Discard bo->map_count

i965: Discard bo->map_count

Supposedly we were keeping a reference count for the number of users of
a mapping so that we could use valgrind to detect access to the map
outside of the valid section. However, we were incrementing the counter
only when first creating the cached mapping but decrementing on every
unmap. The bo->map_count tracking was wrong and so the debugging code
was completely useless.

v2 (Ken): Separate out atomic compare and swap optimization.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>