ps3fb: add virtual screen and panning support

ps3fb: Add virtual screen and panning support:
  - The vertical virtual screen size is limited by the amount of memory
    reserved for ps3fb,
  - The horizontal virtual screen size is limited to the fullscreen width,
  - Advertise that we support panning, so fbcon will use it if the virtual
    screen is enabled.
    Enabling a virtual screen (using `fbset -vyres nnn') can speed up text
    console scrolling by a factor of 10-15, depending on the video mode.

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>