DRI2: Implemented double buffering when using the hardware overlay

That's the right thing to do and fixes issues such as
    https://github.com/ssvb/xf86-video-sunxifb/issues/6

As a result, now the framebuffer size may need to be larger in
order to accomodate two DRI2 buffers in the offscreen part of
the framebuffer. The users of sunxi hardware are advised to
increase the value of fb0_framebuffer_num variable in fex file
to 3 for 32bpp mode and to 5 for 16bpp mode.

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