drm/tinydrm/mi0283qt: Let the display pipe handle power

It's better to leave power handling and controller init to the
modesetting machinery using the simple pipe .enable and .disable
callbacks. Remove unused mipi_dbi_pipe_enable().

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: David Lechner <david@lechnology.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180110185940.53841-6-noralf@tronnes.org