drm/modes: Use strscpy() to copy command-line mode name

The mode name in struct drm_cmdline_mode can hold 32 characters at most,
which can easily get overrun. Switch to strscpy() to prevent such a
thing.

Reported-by: coverity-bot <keescook+coverity-bot@chromium.org>
Addresses-Coverity-ID: 1527354 ("Security best practices violations")
Fixes: a7ab1553 ("drm/modes: Switch to named mode descriptors")
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20221128081938.742410-2-maxime@cerno.tech


Signed-off-by: Maxime Ripard <maxime@cerno.tech>