drm/probe-helper: Document drm_helper_hpd_irq_event() return value

The documentation of the drm_helper_hpd_irq_event() function didn't
document the value that function was returning. Add that part as well.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210914101724.266570-1-maxime@cerno.tech