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 [1]
Link: KSPP/linux#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>

drivers/gpu/drm/drm_modes.c

@@ -2617,8 +2617,7 @@ void drm_mode_convert_to_umode(struct drm_mode_modeinfo *out,
 		break;
 	}
 
-	strncpy(out->name, in->name, DRM_DISPLAY_MODE_LEN);
-	out->name[DRM_DISPLAY_MODE_LEN-1] = 0;
+	strscpy_pad(out->name, in->name, sizeof(out->name));
 }
 
 /**
@@ -2659,8 +2658,7 @@ int drm_mode_convert_umode(struct drm_device *dev,
 	 * useful for the kernel->userspace direction anyway.
 	 */
 	out->type = in->type & DRM_MODE_TYPE_ALL;
-	strncpy(out->name, in->name, DRM_DISPLAY_MODE_LEN);
-	out->name[DRM_DISPLAY_MODE_LEN-1] = 0;
+	strscpy_pad(out->name, in->name, sizeof(out->name));
 
 	/* Clearing picture aspect ratio bits from out flags,
 	 * as the aspect-ratio information is not stored in