drm: zynqmp_dp: Don't retrain the link in our IRQ

Retraining the link can take a while, and might involve waiting for
DPCD reads/writes to complete. In preparation for unthreading the IRQ
handler, move this into its own work function.

Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240809193600.3360015-4-sean.anderson@linux.dev