tridentfb: Blade3D clock fixes

This patch fixes following problems:
- does not allow the m parameter to reach 0 as
  it locks the graphics core (power cycle needed)
- for the newer chips (with new clock registers)
  does not allow of n / m ratio below 4 as it gives
  unstable image on the Blade3D core
- extend shift parameter (k) range  to 2 for the newer
  chips to cope with the n /m >= 4 limit at low resolution
  (bandwidth) modes
- prefer modes with higher n / m ratio (higher k values)

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>