HUGEMEM: rename to BIGMEM and add 8 MB to HEAP_TILER again

Some apps were crashing because the available
memory for video capturing was too low.
drivers/video/omap2/dss/Kconfig / BIGMEM: fix derp that had no real consequences
tuna: BIGMEM: increase VRAM to stock 16MB again

... to test if this fixes a couple of issues.