msm: mdss: Allow WB to use first available control path

WB timeouts are seen when WB is connected after 4k HDMI
use case. This was seen when WB is using the same Mixer
which was earlier used by HDMI, with 3D Mux enabled.
Since 3D Mux registers are double buffered, the best way
to clear control operations is to re-use the control path.
Hence allowing WB to choose the first available control
path.

Change-Id: Ie599a6d58f461ee7905402bf0a948ac829d8de12
Signed-off-by: Krishna Chaitanya Devarakonda <kdevarak@codeaurora.org>