i965: fix memory leak in context/renderbuffer region management

i965: fix memory leak in context/renderbuffer region management

A temporary change to the intelMakeCurrent() function to make
it work with frame buffer objects causes the static regions
associated with the context (the front_region, back_region,
and depth_region) to take on an additional reference, with
no corresponding release.  This causes a memory leak if a
program repeatedly creates and destroys contexts.

The fix is the corresponding hack, to unreference these
regions when the context is deleted, but only if the
framebuffer objects are still present and the same
regions are still referenced within.

Both sets of code have comment blocks referring to each
other.