drm/amd/display: dc_validate_ctx refocunt fixes.

In dc_resource_validate_ctx_copy_construct don't override dst
context refcount.

Remove extra retain to new ctx in dc_update_surfaces_and_stream

Signed-off-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>