drm/mipi-dsi: consider low power transmission

This patch adds a new flag, MIPI_DSI-MODE_LPM, to transmit data
in low power. With this flag, msg.flags has MIPI_DSI_MSG_USE_LPM
so that host driver of each SoC can clear or set relevant register
bit for low power transmission.

All host drivers shall support continuous clock behavior on the
Clock Lane, and optionally may support non-continuous clock behavior.
Both of them can transmit data in high speed of low power.

With each clock behavior, non-continuous or continuous clock mode,
host controller will transmit data in high speed by default so if
peripheral wants to receive data in low power, the peripheral driver
should set MIPI_DSI_MODE_LPM flag.

Signed-off-by: Inki Dae <inki.dae@samsung.com>