ps3fb: video memory size cleanups

- Limit video memory size to avoid crossing a 256 MiB boundary in IOIF space.
- Pass the actual amount of video memory used to lv1_gpu_memory_allocate().

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>