drm/i915/bios: refactor parse general definitions

Early return on failures. Rename the variable for later merging with
parse_device_mappings().

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/785abb904a572752fec68d90d34efeb67774dc1f.1506586821.git.jani.nikula@intel.com