Change dsi_vc_config_l4/vp() to loop for the VC_BUSY flag to change, and
return an error if it fails.

Busy looping is bad, but there's no interrupt that can be used for all the
cases where VC can be busy. So the caller should first try to make sure
that the VC is not busy, if possible, and then call dsi_vc_config_l4/vp().

Most notable case when the caller cannot be sure if the VC is busy is
after frame has been sent. Usually DSI buffers have been emptied until we
need to reconfig the VC, but in some rare cases the VC can still be busy,
and this patch will handle that case.
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@nokia.com>