drm/amd/display: Check stream before comparing them

[WHAT & HOW]
amdgpu_dm can pass a null stream to dc_is_stream_unchanged. It is
necessary to check for null before dereferencing them.

This fixes 1 FORWARD_NULL issue reported by Coverity.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>