drm/tinydrm/mipi-dbi: Add mipi_dbi_enable_flush()

Add and use a function for enabling, flushing and turning on backlight.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: David Lechner <david@lechnology.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180110185940.53841-4-noralf@tronnes.org