drm/i915/bios: add an enum for BDB block IDs

Better grouping, better semantics for find_section(). No functional
changes.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
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/192cc8a45cb5c36ccbde25a725df135793a4263f.1559308269.git.jani.nikula@intel.com