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>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_crc.c

@@ -75,6 +75,11 @@ int amdgpu_dm_crtc_set_crc_source(struct drm_crtc *crtc, const char *src_name)
 		return -EINVAL;
 	}
 
+	if (!stream_state) {
+		DRM_ERROR("No stream state for CRTC%d\n", crtc->index);
+		return -EINVAL;
+	}
+
 	/* When enabling CRC, we should also disable dithering. */
 	if (source == AMDGPU_DM_PIPE_CRC_SOURCE_AUTO) {
 		if (dc_stream_configure_crc(stream_state->ctx->dc,