pvr2fb: update Documentation/fb/pvr2fb.txt

The current version is very old and does not correctly specify how to
set the video mode.

Signed-off by: Adrian McMenamin <adrian@mcmen.demon.co.uk>
Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>