X.org: DBE should validate front and back buffers separately. RHEL 5.2

Issue

• miDbePositionWindow allocates two pixmaps: a front buffer, and a back buffer. If the buffers are supposed to be initialized, it validates a GC against the front buffer, then uses it to fill and/or copy both the front buffer *and* the back buffer, without revalidating.  If the acceleration architecture needs different GC funcs for the two pixmaps -- for example if allocation of the front buffer exhausted video memory -- then this can cause crashes because the GC is not validated for the back buffer pixmap.