drm/i915: Extract opregion vbt presence check

We want to later change intel_opregion_get_vbt to duplicate the vbt
memory if present, which would be an overkill when we just want to
peek into the presence of opregion vbt. Carve out the presence check
into its own function to use in places where only the presence of vbt
is required.

Suggested-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240228213235.2495611-5-radhakrishna.sripada@intel.com