Allow arbitrary numbers of columns for VNC server screen resolution.

The prior code only allowed multiples of 32 for the
numbers of columns. Remove this restriction to allow
a forthcoming UEFI firmware update to allow arbitrary
x,y resolutions.

(the code for handling rows already supported non mult-32 values)

Reviewed by: Leon Dang (original author)
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D15274