i965: Issue performance warnings when copying the program cache BO.

i965: Issue performance warnings when copying the program cache BO.

We don't really want unnecessary buffer copying, so it'd be nice to know
when it's happening.

v2: Drop stall warnings when doing a read-only CPU mapping of the cache
    BO.  The GPU also uses it in a read-only fashion, so there won't be
    any stalls, even though the buffer is busy.  (Thanks to Chris Wilson
    for catching this mistake.)

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net> [v1]