ps3fb: fix possible overlap of GPU command buffer and frame buffer

ps3fb: In the case of non-fullscreen video modes, there was a partial overlap
of the GPU command buffer and the frame buffer. Fix and cleanup various issues
with overlap and alignment:
  - Move the GPU command buffer from the beginning to the end of video memory
  - Exclude the GPU command buffer from the actual frame buffer memory
  - Align the start of the virtual frame buffer to PAGE_SIZE instead of to 64
    KiB, and don't waste memory if it's already aligned (for fullscreen modes)
  - Take into account the alignment when checking memory requirements and
    maximum number of frames
  - Make sure fb_fix_screeninfo.smem_start always points to the virtual frame
    buffer start, so we don't have to compensate for that in ps3fb_mmap()

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>