drm/i915: enum i915_power_well_id is not proper kernel-doc

Revert to a normal comment, as the enum isn't properly documented
anyway.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f2c44a0aa00ea7d9b71e7a3183a7507f98811146.1502368010.git.jani.nikula@intel.com