Tuna: add option to reclaim ~55 MB of reserved RAM (BIGMEM)

Reclaim 50M of RAM from HEAP_TILER and 5M from the OMAP framebuffer.
If we reclam max 50 MB from HEAP_TILER, 1080p mode is still working.

Credits to mpokwsths for the idea,  boype for making it a more organized and BigBrother1984 for the safe reclaim value.

Signed-off-by: Dan Trifan <jimsth@gmail.com>
defconfig: enable TUNA_BIGMEM

Signed-off-by: Dan Trifan <jimsth@gmail.com>
version 0.7.6 exp

Signed-off-by: Dan Trifan <jimsth@gmail.com>