fbdev: drop custom atoi from drivers/video/modedb.c

Kernel has simple_strtol() implementation which could be used as atoi().

Signed-off-by: Andy Shevchenko <ext-andriy.shevchenko@nokia.com>
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>