drm/i915/bios: Extract vbt_signature[]

Replace the three hand rolled "$VBT"s with a vbt_signature[]
to avoid accidents.

v2: Include terminating '\0' for safety (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240923152453.11230-6-ville.syrjala@linux.intel.com