drm/amd/display: fix a missing null check on a failed kzalloc

Currently the allocation of config may fail and a null pointer
dereference on config can occur.  Fix this by added a null
check on a failed allocation of config.

Addresses-Coverity: ("Dereference null return")
Fixes: c2cd9d04 ("drm/amd/display: Hook up calls to do stereo mux and dig programming to stereo control interface")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>