drm/amdgpu: utilize subconnector property for DP through atombios

Since DP-specific information is stored in driver's structures, every
driver needs to implement subconnector property by itself.

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>
Reviewed-by: Emil Velikov <emil.velikov@collabora.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/1587732655-17544-4-git-send-email-jeevan.b@intel.com