drm/rockchip: vop: add rk3188 hs_start interrupt as dsp_hold equivalent

The hs_start interrupt on rk3188 fires at the start of a new frame, so
serves essentially the same purpose as the dsp_hold_valid irq in checking
when the last frame got delivered when going to standby. So define it
to fix a hang on atomic_disable of the vop because the completion never
really completed before.

Fixes: 428e15cc ("drm/rockchip: vop: add rk3188 vop definitions")
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Sandy Huang <hjc@rock-chips.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180923123730.14706-1-heiko@sntech.de