drm/tinydrm: Fix fbdev pixel format

Due to copy/paste error, the fbdev format was changed to 32bpp = XRGB8888
which is an emulated format for the RGB565 drivers. Revert to to using the
fallback which is dev->mode_config.preferred_depth for the drivers that
set it or 32bpp for those that don't (repaper, st7586).

Fixes: 3eba3922 ("drm/tinydrm: Drop using tinydrm_device")
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190410124345.25945-1-noralf@tronnes.org