drm/rockchip: dw-mipi-dsi: respect message flags

Instead of always sending commands in LP mode, respect the
MIPI_DSI_MSG_USE_LPM flag to decide how to send each message.  Also
request acks if MIPI_DSI_MSG_REQ_ACK is set.

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/20170224125506.21533-9-john@metanate.com