drm/amd/display: added NULL check at start of dc_validate_stream

[Why]
prevent invalid memory access

[How]
check if dc and stream are NULL

Co-authored-by: winstang <winstang@amd.com>
Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Acked-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Signed-off-by: winstang <winstang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>