drm/amd/display: Guard against null stream_state in set_crc_source

[Why]

The igt@kms_plane@pixel-format-pipe tests can create a sequence where
stream_state is NULL during amdgpu_dm_crtc_set_crc_source which results
in a null pointer dereference.

[How]

Guard against stream_state being NULL before accessing its fields. This
doesn't fix the root cause of the issue so a DRM_ERROR is generated
to still fail the tests.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: David Francis <David.Francis@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>