drm/poll-helper: merge drm_kms_helper_poll_disable() and _fini()

Merge drm_kms_helper_poll_disable() and drm_kms_helper_poll_fini() code
into a common helper function.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20221102180705.459294-2-dmitry.baryshkov@linaro.org