mali: detect and workaround mismatch between back and front buffers

After window creation or resize, the mali blob on the client side
requests two dri2 buffers (for back and front) from the ddx. The
problem is that the 'swap' and 'get_buffer' operations are executed
out of order relative to each other and we may have different
possible patterns of dri2 communication:

1. swap swap swap swap get_buffer swap get_buffer swap swap ...
2. swap swap swap get_buffer swap swap get_buffer swap swap ...

A major annoyance is that both mali blob on the client side and
the ddx driver in xserver need have the same idea about which one
of there two buffers goes to front and which goes to back. Older
commit https://github.com/ssvb/xf86-video-fbturbo/commit/30b4ca27d1c4
tried to address this problem in a mostly empirical way and managed
to solve it at least for the synthetic test gles-rgb-cycle-demo and
for most of the real programs (such as Qt5 applications, etc.)

However appears that this heuristics is not 100% reliable in all
cases. The Extreme Tux Racer game run in glshim manages to trigger
the back and front buffers mismatch. Which manifests itself as
erratic penguin movement.

This patch adds a special check, which now randomly samples certain
bytes from the dri2 buffers to see which one of them has been
modified by the client application between buffer swaps. If we see
that the rendering actually happens to the front buffer instead of
the back buffer, then we just change the roles of these buffers.

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