drm: Helper to read max bits per component

Helper routine to read out maximum supported bits per
component for DisplayPort legay converters.

v2: Return early if detailed port cap info is not available.
    Replace if-else ladder with switch-case (Ville)

Reviewed-by: Jim Bride <jim.bride@linux.intel.com>
Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1473419458-17080-5-git-send-email-mika.kahola@intel.com
diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index 60dd9dc..f3d1424 100644
--- a/include/drm/drm_dp_helper.h
+++ b/include/drm/drm_dp_helper.h
@@ -817,6 +817,8 @@
 int drm_dp_link_configure(struct drm_dp_aux *aux, struct drm_dp_link *link);
 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]);
 
 void drm_dp_aux_init(struct drm_dp_aux *aux);
 int drm_dp_aux_register(struct drm_dp_aux *aux);