drm/doc: Update errno handbook

We recently bikeshedded this to a different flavour, but forgot to
update the recommendations.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181019084311.23836-1-daniel.vetter@ffwll.ch

Documentation/gpu/drm-uapi.rst

@@ -197,6 +197,9 @@ EPERM/EACCESS:
         difference between EACCESS and EPERM.
 
 ENODEV:
+        The device is not (yet) present or fully initialized.
+
+EOPNOTSUPP:
         Feature (like PRIME, modesetting, GEM) is not supported by the driver.
 
 ENXIO: