drm/panel: Enable DSI transactions on the RPi panel.

It turns out that I had just mistaken what type of write the register
writes were supposed to be, using DCS instead of generic long writes.

Switching to transactions instead of using the atmel as a bridge also
seems to resolve the sparkling pixels problem I've had.

Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: 2f733d61

 ("drm/panel: Add support for the Raspberry Pi 7" Touchscreen.")
Link: https://patchwork.freedesktop.org/patch/msgid/20171031193258.17373-2-eric@anholt.net
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
Acked-by: Thierry Reding <treding@nvidia.com>