drm/amd/display: cleanup the code a bit

In function dc_sink_destruct, kfree will check pointer, no need
to check again.
This change is to cleanup the code a bit.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Bernard Zhao <bernard@vivo.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>