drm/vkms: Use drmm_mode_config_init()

Use drmm_mode_config_init() instead of drm_mode_config_init(), as it allows
us to assure that the resource will be properly cleaned.

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20230116205800.1266227-2-mcanal@igalia.com