drm/tinydrm: Move tinydrm_machine_little_endian()

The tinydrm helper is going away so move it into the only user mipi-dbi.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190719155916.62465-9-noralf@tronnes.org