drm/msm/dp: attach the DP subconnector property

While developing and testing the commit bfcc3d8 ("drm/msm/dp:
support setting the DP subconnector type") I had the patch [1] in my
tree. I haven't noticed that it was a dependency for the commit in
question. Mea culpa.

Since the patch has not landed yet (and even was not reviewed)
and since one of the bridges erroneously uses USB connector type instead
of DP, attach the property directly from the MSM DP driver.

This fixes the following oops on DP HPD event:

 drm_object_property_set_value (drivers/gpu/drm/drm_mode_object.c:288)
 dp_display_process_hpd_high (drivers/gpu/drm/msm/dp/dp_display.c:402)
 dp_hpd_plug_handle.isra.0 (drivers/gpu/drm/msm/dp/dp_display.c:604)
 hpd_event_thread (drivers/gpu/drm/msm/dp/dp_display.c:1110)
 kthread (kernel/kthread.c:388)
 ret_from_fork (arch/arm64/kernel/entry.S:858)

[1] https://patchwork.freedesktop.org/patch/555530/

Fixes: bfcc3d8 ("drm/msm/dp: support setting the DP subconnector type")
Reviewed-by: Abhinav Kumar <[email protected]>
Signed-off-by: Dmitry Baryshkov <[email protected]>
Tested-by: Jessica Zhang <[email protected]> # SC7280
Reviewed-by: Johan Hovold <[email protected]>
Tested-by: Johan Hovold <[email protected]>
Patchwork: https://patchwork.freedesktop.org/patch/564286/
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Abhinav Kumar <[email protected]>

drivers/gpu/drm/msm/dp/dp_drm.c

@@ -345,6 +345,9 @@ struct drm_connector *dp_drm_connector_init(struct msm_dp *dp_display, struct dr
 	if (IS_ERR(connector))
 		return connector;
 
+	if (!dp_display->is_edp)
+		drm_connector_attach_dp_subconnector_property(connector);
+
 	drm_connector_attach_encoder(connector, encoder);
 
 	return connector;