tridentfb: fix hi-color modes for TGUI 9440

The TGUI 9440 requires doubling clock for 16bpp (hi-color) modes.

The patch also moves back enable_mmio() call to the right position.

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>