drm/i915/bios: remove an unnecessary temp variable

Prepare for merging parse_device_mapping() into
parse_general_definitions(). No functional changes.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f1c3621e2622f4debdfb4a2f5c1959845754ac04.1506586821.git.jani.nikula@intel.com