drm/amd/display: fix a dereference of a NULL pointer

[why&how]
In some platform out_transfer_func may not be popualted. We need to check
for null before dereferencing it.

Fixes: d2dea1f1 ("drm/amd/display: Generalize new minimal transition path")
Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>