drm/sysfb: efidrm: Support module builds

Make CONFIG_DRM_EFIDRM a tristate to enable module builds.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20250408091837.407401-3-tzimmermann@suse.de