drm/i915/uapi: Add kerneldoc for engine class enum

We'll be adding a new type of engine soon.  Let's document the existing
engine classes first to help make it clear what each type of engine is
used for.

Cc: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220428041926.1483683-2-matthew.d.roper@intel.com