drm/amd/display: Keep PHY active for dp config

[Why]
Current hotplug sequence causes temporary hang at the re-entry of the
optimized power state.

[How]
Keep a PHY active when detecting DP signal + DPMS active

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Agustin Gutierrez <agustin.gutierrez@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>