drm/i915: Document the sets of bits in the driver features block

Add a few comment documenting the sets of bits in the driver
features block. Might make it a bit easier to check against
the spec.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220715202044.11153-8-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>