drm: modes: Revert cc344980 "replace simple_strtoul by kstrtouint"

My latest commit introduce some case where a valid mode, could be
rejected.
simple_strtox functions stop at first non-digit character, but kstrtox
not.
So args like "video=HDMI-A-1:720x480-16@60" will be reject when checking
16@.

Discussions about this change comes to the conclusion that the best
solution is to revert my commit cc344980.

Signed-off-by: LABBE Corentin <clabbe.montjoie@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449838739-29969-1-git-send-email-clabbe.montjoie@gmail.com