drm/rockchip: dw-mipi-dsi: only request HS clock when required

Requesting the HS clock from the PHY before we initialize it causes an
invalid signal to be sent out since the input clock is not yet
configured.  The PHY databook suggests only asserting this signal when
performing HS transfers, so let's do that.

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/20170224125506.21533-10-john@metanate.com