drm/i915/dp: Read DP descriptor for eDP and LSPCON too

As for external DP sink and branch devices read and print the DP
descriptor for eDP and LSPCON devices as well to aid debugging.

v2:
- Split out this change to a separate patch. (Jani)

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1477326811-30431-7-git-send-email-imre.deak@intel.com
diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index b958326..20fb6e7 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -1468,6 +1468,7 @@ static inline unsigned int intel_dp_unused_lane_mask(int lane_count)
 }
 
 bool intel_dp_read_dpcd(struct intel_dp *intel_dp);
+bool intel_dp_read_desc(struct intel_dp *intel_dp);
 
 /* intel_dp_aux_backlight.c */
 int intel_dp_aux_init_backlight_funcs(struct intel_connector *intel_connector);