drm/i915/bios: merge parse_device_mapping() into parse_general_definitions()

They're both parsing the same block, and there's no need for them to be
split. The former also benefits from the range checks in the latter.

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/64a292606ecbb0b8602e6c5523c5746573ec3944.1506586821.git.jani.nikula@intel.com