drm/rockchip/dsi: enable the grf clk before writing grf registers

For RK3399, the grf clk should be enabled before writing grf registers,
otherwise the register value can not be changed.

Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1490147691-4489-4-git-send-email-zyw@rock-chips.com