s3fb: do not allow incorrect pixclock settings

This patch adds check if selected pixclock is valid (is in the PLL range).

Previously, if the pixclock could not be set, the new mode resolution was set
but pixclock was not set which led to incorrect timings sent to monitor.

[adaplas]
Fixed a few misplaced curly braces.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>