drm/rockchip/dsi: add dw-mipi power domain support

Reference the power domain incase dw-mipi power down when
in use.

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: https://patchwork.freedesktop.org/patch/msgid/1487577744-2855-8-git-send-email-zyw@rock-chips.com