drm/edid: kerneldoc for is_hdmi2_sink

Also some breadcrumbs for how exactly to find this. Probably should
pass drm_connector * or at least drm_display_info * to that function
instead. But drm_hdmi_avi_infoframe_quant_range probably also wants
drm_connector_state (and the hdmi stuff moved into that), so this is a
bit more work.

v2: Fix typo (Alex).

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Fixes: 9271c0ca ("drm/edid: Don't send non-zero YQ in AVI infoframe for HDMI 1.x sinks")
Acked-by: NAlex Deucher <alexander.deucher@amd.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171214203054.20141-2-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_edid.c

@@ -4871,6 +4871,11 @@ EXPORT_SYMBOL(drm_hdmi_avi_infoframe_from_display_mode);
  * @mode: DRM display mode
  * @rgb_quant_range: RGB quantization range (Q)
  * @rgb_quant_range_selectable: Sink support selectable RGB quantization range (QS)
+ * @is_hdmi2_sink: HDMI 2.0 sink, which has different default recommendations
+ *
+ * Note that @is_hdmi2_sink can be derived by looking at the
+ * &drm_scdc.supported flag stored in &drm_hdmi_info.scdc,
+ * &drm_display_info.hdmi, which can be found in &drm_connector.display_info.
  */
 void
 drm_hdmi_avi_infoframe_quant_range(struct hdmi_avi_infoframe *frame,