drm/amd/display: Check stream_status before it is used

[WHAT & HOW]
dc_state_get_stream_status can return null, and therefore null must be
checked before stream_status is used.

This fixes 1 NULL_RETURNS issue reported by Coverity.

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