tridentfb: fix timing calculations

Fix broken timings calculations. This patch helps with following
problems:
 - no left part of screen visible (up to half of the screen)
 - monitor's frequencies are not the ones intended for selected modes
 - if mode with resoultion y > 1024 is selected at least once then
   all modes with y < 1024 are "out of sync" (no display)

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>