drm/dp: Pimp drm_dp_downstream_max_bpc()

Deal with more cases in drm_dp_downstream_max_bpc():
- DPCD 1.0 -> assume 8bpc for non-DP
- DPCD 1.1+ DP (or DP++ with DP sink) -> allow anything
- DPCD 1.1+ TMDS -> check the caps, assume 8bpc if the value is crap
- anything else -> assume 8bpc

v2: Use Returns: for kdoc (Lyude)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200904115354.25336-8-ville.syrjala@linux.intel.com
Reviewed-by: Lyude Paul <lyude@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>

include/drm/drm_dp_helper.h

@@ -1646,10 +1646,14 @@ bool drm_dp_downstream_is_tmds(const u8 dpcd[DP_RECEIVER_CAP_SIZE],
 int drm_dp_downstream_max_clock(const u8 dpcd[DP_RECEIVER_CAP_SIZE],
 				const u8 port_cap[4]);
 int drm_dp_downstream_max_bpc(const u8 dpcd[DP_RECEIVER_CAP_SIZE],
-			      const u8 port_cap[4]);
+			      const u8 port_cap[4],
+			      const struct edid *edid);
 int drm_dp_downstream_id(struct drm_dp_aux *aux, char id[6]);
-void drm_dp_downstream_debug(struct seq_file *m, const u8 dpcd[DP_RECEIVER_CAP_SIZE],
-			     const u8 port_cap[4], struct drm_dp_aux *aux);
+void drm_dp_downstream_debug(struct seq_file *m,
+			     const u8 dpcd[DP_RECEIVER_CAP_SIZE],
+			     const u8 port_cap[4],
+			     const struct edid *edid,
+			     struct drm_dp_aux *aux);
 enum drm_mode_subconnector
 drm_dp_subconnector_type(const u8 dpcd[DP_RECEIVER_CAP_SIZE],
 			 const u8 port_cap[4]);