drm/mcde: Fix DSI transfers

There were bugs in the DSI transfer (read and write) function
as it was only tested with displays ever needing a single byte
to be written. Fixed it up and tested so we can now write
messages of up to 16 bytes and read up to 4 bytes from the
display.

Tested with a Sony ACX424AKP display: this display now self-
identifies and can control backlight in command mode.

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: 5fc537bf

 ("drm/mcde: Add new driver for ST-Ericsson MCDE")
Reviewed-by: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190903170804.17053-1-linus.walleij@linaro.org