drm/bridge: Avoid uninitialized variable warning

This code works, but technically it uses "num_in_bus_fmts" before it
has been initialized so it leads to static checker warnings and probably
KMEMsan warnings at run time.  Initialize the variable to zero to
silence the warning.

Fixes: f32df58a ("drm/bridge: Add the necessary bits to support bus format negotiation")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/YrrIs3hoGcPVmXc5@kili