drm/amd/display: Fix NULL pointer dereference

drm/amd/display: Fix NULL pointer dereference

[Why]
On a mst branch with multi display setup, dc context is obselete
after updating the first stream. Referencing the same dc context
for the next stream update to fetch dc pointer leads to NULL
pointer dereference.

[How]
Get the dc pointer from the link rather than context.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Signed-off-by: Meenakshikumar Somasundaram <meenakshikumar.somasundaram@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>