drm/modes: Fix unterminated strncpy

strncpy(dest, src, strlen(src)) leads to unterminated
dest, which is dangerous.
Fix it by using strscpy.

Fixes: 3aeeb13d ("drm/modes: Support modes names on the command line")
Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190731020140.3529-1-hslester96@gmail.com