drm/bridge: tc358768: Separate indirect register writes

drm/bridge: tc358768: Separate indirect register writes

Some registers can only be written indirectly, using DSI_CONFW register.
We don't have many uses for those registers (in fact, only DSI_CONTROL
is currently written), but the code to do those writes inline is a bit
confusing.

Add a new function, tc358768_confw_update_bits() which can be used to
write the bits indirectly. Only DSI_CONTROL is currently supported.

Tested-by: João Paulo Gonçalves <joao.goncalves@toradex.com> # Toradex Verdin AM62
Reviewed-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Link: https://patch.msgid.link/20260311-tc358768-v2-3-e75a99131bd5@ideasonboard.com
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>