tridentfb: various pixclock and timing improvements

This patch fixes few issues related to timings and pixclock generation:

 - disallow the pixclocks with numerator lower than
   double denominator. This fixes display instability
   for some modes.
 - choose the pixelclock with the highest
   numerator and denominator values. This improve
   image quality and fixes display instability
   for some modes.
 - make interlaced modes work.
 - set synchronization pulses polarization
   correctly.
 - horizontal synchronization timing are now
   the same as generated by X.

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>