drm/probe-helper: Check epoch counter in output_poll_execute()

drm_helper_hpd_irq_event() checks the epoch counter to determine
connector status change. This was introduced in
commit 5186421c ("drm: Introduce epoch counter to drm_connector").
Do the same for output_poll_execute() so it can detect other changes
beside connection status value changes.

v2:
- Add Fixes tag (Daniel)

Fixes: 5186421c

 ("drm: Introduce epoch counter to drm_connector")
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210313112545.37527-3-noralf@tronnes.org