drivers/gpu/drm/drm_fb_helper.c: don't use private implementation of atoi()

Kernel has simple_strtol() which would be used as atoi().

This is quite the same fix as in 2cb96f86
("fbdev: drop custom atoi from drivers/video/modedb.c") because code in
drivers/gpu/drm/drm_fb_helper.c is based on drivers/video/modedb.c.

Signed-off-by: Andy Shevchenko <ext-andriy.shevchenko@nokia.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>