Avoid division by zero in  atmel_lcdfb_check_var() function.

If pixclock is not specified while passing a var structure in
the check_var() funtion, a division by zero occurs (when
translating pixclock to KHz).

This patch adds a checking of this value and try to choose a
video mode in the modelist.

The mode found in the probe function in added to the modelist.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Haavard Skinnemoen <hskinnemoen@atmel.com>
Cc: Andrew Victor <linux@maxim.org.za>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>