drm/rect: update kerneldoc for drm_rect_clip_scaled()

This was forgotten in f96bdf56 ("drm/rect: Handle rounding errors
in drm_rect_clip_scaled, v3.")

Spotted while reviewing patches from Ville touching this area.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Fixes: f96bdf56 ("drm/rect: Handle rounding errors in drm_rect_clip_scaled, v3.")
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Benjamin Gaignard <benjamin.gaignard@st.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191126145213.380079-1-daniel.vetter@ffwll.ch