drm/amd/display: Disallow entering PSR when panel is disconnected

[WHY]
The dGPU cannot enter PSR when it is not connected to a panel.

[HOW]
Added a check to dc_link_set_psr_allow_active
which returns early if panel is disconnected.

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: Max Erenberg <merenber@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>