DRI2: CPU copy fallback path does not drop half of the frames anymore

The recent commit 9e0a87319b90e3e364fde7cffd24662926f5a4fa (its part
that suppressed buffers reuse in the Xorg DRI2 framework) introduced
a regression. Half of the frames stoppped reaching the screen on
the CPU copy fallback path because the Mali blob now ended up
rendering them to the "wrong" buffer.

It just confirms that we need to completely move from the standard
DRI2 framework in the Xorg server to our own buffers bookkeeping
logic. This patch fixes the regression by introducing a single UMP
buffer per window, which is shared between back and front DRI2
buffers. We can do this because double buffering does not make much
sense on the fallback path at the moment (we can't set scanout from
this buffer and anyway have to copy this data elsewhere immediately
after we get it from Mali).

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>