pm2fb: memclock setting corrections

This patch disables a memory clock setting if a board has been initialized by
BIOS.  This allows using the memory clock set by manufacturer of the board.

This patch also sets default clock for 3dlabs Permedia 2V reference board's
clock to 75MHz (BIOS setting for EONtronic Permedia 2Vboard), because the
default 83MHz can be too high.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
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>