drm: xlnx: zynqmp_dpsub: fix hotplug detection

drm_kms_helper_poll_init needs to be called after zynqmp_dpsub_kms_init.
zynqmp_dpsub_kms_init creates the connector and without it we don't
enable hotplug detection.

Fixes: eb2d64bf ("drm: xlnx: zynqmp_dpsub: Report HPD through the bridge")
Cc: stable@vger.kernel.org
Signed-off-by: Steffen Dirkwinkel <s.dirkwinkel@beckhoff.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241028134218.54727-1-lists@steffen.cc