drm/vc4: hvs: Reset muxes at probe time

commit  a2c2fd1b68710016a6e1be2bcd07a13bf198097e from
https://github.com/raspberrypi/linux.git rpi-5.15.y

By default, the HVS driver will force the HVS output 3 to be muxed to
the HVS channel 2. However, the Transposer can only be assigned to the
HVS channel 2, so whenever we try to use the writeback connector, we'll
mux its associated output (Output 2) to the channel 2.

This leads to both the output 2 and 3 feeding from the same channel,
which is explicitly discouraged in the documentation.

In order to avoid this, let's reset all the output muxes to their reset
value.

Fixes: 87ebcd42

 ("drm/vc4: crtc: Assign output to channel automatically")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Meng Li <Meng.Li@windriver.com>