This takes care of 2 TODOs in this driver, by using the common DSI
packet-marshalling code instead of our custom short/long write code.
This both saves us some duplicated code and gets us free support for
command types that weren't already part of our switch block (e.g.,
MIPI_DSI_GENERIC_LONG_WRITE).

The code logic stays mostly intact, except that it becomes unnecessary
to split the short/long write functions, and we have to copy data a bit
more.

Along the way, I noticed that loop bounds were a little odd:

	while (DIV_ROUND_UP(len, pld_data_bytes))

This really was just supposed to be 'len != 0', so I made that more
clear.

Tested on RK3399 with some pending refactoring patches by Nickey Yang,
to make the Rockchip DSI driver wrap this common driver.
Signed-off-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Philippe Cornu <philippe.cornu@st.com>
Tested-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180109203248.139249-1-briannorris@chromium.org