drm/amd/display: fix the return of the uninitialized value in ret

Currently if stream->signal is neither SIGNAL_TYPE_DISPLAY_PORT_MST or
SIGNAL_TYPE_DISPLAY_PORT then variable ret is uninitialized and this is
checked for > 0 at the end of the function.  Ret should be initialized,
I believe setting it to zero is a correct default.

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: bd0c064c

 ("drm/amd/display: Add return code instead of boolean for future use")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>