dri3: Prefer the last chosen back when finding a new one.

dri3: Prefer the last chosen back when finding a new one.

With the buffer_age code, I need to be able to potentially call this more
than once per frame, and it would be bad if a new special event showing up
meant I chose a different back mid-frame.  Now, once we've chosen a back
for the frame, another find_back will choose it again since we know that
it won't have ->busy set until swap.

Note that this makes find_back return a buffer id instead of a backbuffer
index.  That's kind of a silly distinction anyway, since it's an identity
mapping between the two (it's the front buffer that is at an offset).

Reviewed-By: Adel Gadllah <adel.gadllah@gmail.com>