drm/rockchip: dw-mipi-dsi: prepare panel after phy init

Some panels need to be configured with commands sent over the MIPI link,
which they will do in the prepare hook.  Call this after the PHY has
been initialized so that we are able to send commands to the panel.

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-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/20170224125506.21533-12-john@metanate.com