drm/amd/display: enable audio on DP seamless boot

[Why]
Some external displays with DP can use seamless boot, but their audio endpoints are
only enabled after hotplug.
On boot, these displays inherit UEFI timings without properly initializing audio.

[How]
Enable display audio when seamless booting with an external monitor over DP.

Signed-off-by: Anthony Wang <anthony1.wang@amd.com>
Acked-by: Bindu Ramamurthy <bindu.r@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>