drm/amd/display: Create a temporary scratch dc_link

Create a temporary scratch dc_link for programming purposes
and fix a copy of pipe_ctx on the stack to a pointer reference.

Reviewed-by: Josip Pavic <josip.pavic@amd.com>
Signed-off-by: Aric Cyr <Aric.Cyr@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>