cirrusfb: set MCLK in one place

A memory clock (MCLK) is set at various places.  Move the setting into one
place.

Set the MCLK only for Zorro cards as the x86 cards should be initialized by
BIOS.

Improve handling of the GD5434 (SD64).

Kill one annoying debug output "virtual offset: ...".

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>