tridentfb: y-panning fixes

The Trident cards uses only 20-bit address of screen start in double
words.  This allows addressing for only 4MB of video memory so check this.

Also remove some redundant checks and assignments.

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>