drm/amd/display: Added dc_edp_id_count to dc_context

[Why]
We need to keep track of the number of eDP links to
properly enumerate edp panel control instance.

[How]
Added dc_edp_id_count to dc_context to keep track of
number of eDP links.

Signed-off-by: Jake Wang <haonan.wang2@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>