drm/modes: replace deprecated strncpy with strscpy_pad

drm/modes: replace deprecated strncpy with strscpy_pad

`strncpy` is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.

We should NUL-pad as there are full struct copies happening in places:
|       struct drm_mode_modeinfo umode;
|
|       ...
|               struct drm_property_blob *blob;
|
|               drm_mode_convert_to_umode(&umode, mode);
|               blob = drm_property_create_blob(crtc->dev,
|                                               sizeof(umode), &umode);

A suitable replacement is `strscpy_pad` due to the fact that it
guarantees both NUL-termination and NUL-padding on the destination
buffer.

Additionally, replace size macro `DRM_DISPLAY_MODE_LEN` with sizeof() to
more directly tie the maximum buffer size to the destination buffer:
|       struct drm_display_mode {
|               ...
|       	char name[DRM_DISPLAY_MODE_LEN];

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Cc: Xu Panda <xu.panda@zte.com.cn>
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20231016-strncpy-drivers-gpu-drm-drm_modes-c-v2-1-d0b60686e1c6@google.com
Signed-off-by: Kees Cook <keescook@chromium.org>