drm/mipi_dsi: add flags to DSI messages

This patch adds flags field to mipi_dsi_msg structure and two flags:
- MIPI_DSI_MSG_REQ_ACK - request ACK from peripheral for given message,
- MIPI_DSI_MSG_USE_LPM - use Low Power Mode to transmit message.
The first flag is usually helpful during DSI diagnostic, the second
flag is required by some peripherals during configuration phase.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>