gma500,cdv: Fix the brightness base

Some desktop environments carefully save and restore the brightness
settings from the previous boot.  Unfortunately they don't all check to
see if the range has changed.  The end result is that they restore a
brightness of 100/lots not 100/100.

As the old driver and the non-free GMA36xx driver both use 0-100 we thus
need to go back doing the same thing to avoid users getting a mysterious
black screen after boot.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>