drm/i915/opregion: add intel_opregion_vbt_present() stub for ACPI=n

drm/i915/opregion: add intel_opregion_vbt_present() stub for ACPI=n

The opregion code needs stubs for ACPI=n. Add the missing stub for
intel_opregion_vbt_present().

Reported-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Closes: https://lore.kernel.org/r/20240312120240-afdb1b83-8517-434b-be79-06f41bafd71f@linutronix.de
Fixes: 9d9bb71f3e11 ("drm/i915: Extract opregion vbt presence check")
Cc: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240312115757.683584-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>