drm/amd/display: Fix incorrect DSC instance for MST

drm/amd/display: Fix incorrect DSC instance for MST

[Why] DSC debugfs, such as dp_dsc_clock_en_read,
use aconnector->dc_link to find pipe_ctx for display.
Displays connected to MST hub share the same dc_link.
DSC instance is from pipe_ctx. This causes incorrect
DSC instance for display connected to MST hub.

[How] Add aconnector->sink check to find pipe_ctx.

CC: stable@vger.kernel.org
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>