drm/rockchip: vop: fix vop vsync/hsync polarity

Vop set wrong vsync/hsync polarity, it may cause some
display problem. known problem is that caused HDMI hdcp
authenticate failed, caused pixel offset with hdmi display.
the polarity description at RK3288 TRM doc:
  dsp_vsync_pol
    VSYNC polarity
      1'b0 : negative
      1'b1 : positive
      dsp_hsync_pol
    HSYNC polarity
      1'b0 : negative
      1'b1 : positive

Signed-off-by: Mark Yao <mark.yao@rock-chips.com>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Tested-by: Caesar Wang <wxt@rock-chips.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>