mali: /var/log/Xorg.0.log warning about insufficient framebuffer size

In the case if the framebuffer reservation size is too small for
efficient use of the hardware overlays and zero-copy buffers flipping,
log a hint about fixing this problem in /var/log/Xorg.0.log

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