drm/rockchip/dsi: correct the grf_switch_reg name

For the RK3399, the grf_switch_reg name should be RK3399_GRF_SOC_CON20,
not RK3399_GRF_SOC_CON19.

Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1490147691-4489-5-git-send-email-zyw@rock-chips.com