drm/tinydrm/mipi-dbi: Add poweron-reset functions

Split out common poweron-reset functionality.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: David Lechner <david@lechnology.com>
Tested-by: David Lechner <david@lechnology.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180110185940.53841-5-noralf@tronnes.org