drm/amd/display: Keep PHY active for DP displays on DCN31

[Why & How]
Port of a change that went into DCN314 to keep the PHY enabled
when we have a connected and active DP display.

The PHY can hang if PHY refclk is disabled inadvertently.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Josip Pavic <josip.pavic@amd.com>
Acked-by: Alan Liu <haoping.liu@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>