drm/print: convert debug category macros into an enum

Mostly for improved documentation, convert the debug category macros
into an enum. Drop unused DRM_UT_NONE. Document previously undocumented
categories.

Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Sean Paul <sean@poorly.run>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/96582479e7829d92b89adb805f829e23043ca85c.1572258936.git.jani.nikula@intel.com