drm/sun4i: sun6i_mipi_dsi: Support DSI GENERIC_SHORT_WRITE_2 transfer

Some DSI panels do use GENERIC_SHORT_WRITE_2 transfer protocol to host
DSI driver and which is similar to GENERIC_SHORT_WRITE.

Add support for the same transfer, so-that so-that the panels which are
requesting similar transfer type will process properly.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Tested-by: Merlijn Wajer <merlijn@wizzup.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190512184128.13720-3-jagan@amarulasolutions.com