drm/amdgpu: utilize subconnector property for DP through DisplayManager

Since DP-specific information is stored in driver's structures, every
driver needs to implement subconnector property by itself. Display
Core already has the subconnector information, we only need to
expose it through DRM property.

v2:rebase

v3: renamed a function call

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: David (ChunMing) Zhou <David1.Zhou@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Signed-off-by: Jeevan B <jeevan.b@intel.com>
Signed-off-by: Oleg Vasilev <oleg.vasilev@intel.com>
Tested-by: Oleg Vasilev <oleg.vasilev@intel.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1587732655-17544-5-git-send-email-jeevan.b@intel.com