drm/amd/display: Fix crash on setting VRR with no display connected

[HOW&WHY]
VRR was getting set at the same time
the timing generator would be null when there was no display
connected. Added null check to the timing generator variable
so it does not get referenced if it is null.

Reviewed-by: Harry Vanzylldejong <harry.vanzylldejong@amd.com>
Reviewed-by: Evgenii Krasnikov <Evgenii.Krasnikov@amd.com>
Reviewed-by: Nicholas Choi <Nicholas.Choi@amd.com>
Acked-by: Pavle Kotarac <Pavle.Kotarac@amd.com>
Signed-off-by: Harry VanZyllDeJong <hvanzyll@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>