drm: of: Add drm_of_get_dsi_bus helper function

Add helper function to find DSI host for devices where DSI panel is not
a minor of a DSI bus (such as the Samsung AMS495QA01 panel or the
official Raspberry Pi touchscreen display).

Co-developed-by: Maya Matuszczyk <maccraft123mc@gmail.com>
Signed-off-by: Maya Matuszczyk <maccraft123mc@gmail.com>
Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Reviewed-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230123154603.1315112-2-macroalpha82@gmail.com