drm/panel: simple: Fix the bus format for the Ontat panel

This fixes bad color output.  When I was first testing the device I
had the DPI hardware set to 666 mode, but apparently in the refactor
to use the bus_format information from the panel driver, I failed to
actually update the panel.

Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: e8b6f561 ("drm/panel: simple: Add the 7" DPI panel from Adafruit")
Cc: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180309233332.1769-1-eric@anholt.net