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: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171214203054.20141-2-daniel.vetter@ffwll.ch