drm/amd/display: set new_stream to NULL after release

In dm_update_crtc_state(), the skip_modeset path releases new_stream
via dc_stream_release() but does not set the pointer to NULL.

If a later error (e.g., color management failure) triggers the fail
label, the error path calls dc_stream_release() again on the same
dangling pointer, causing a double release and potential use-after-free.

Fix this by setting new_stream to NULL after the initial release.

Fixes: 9b690ef3c7 ("drm/amd/display: Avoid full modeset when not required")
Signed-off-by: WenTao Liang <vulab@iscas.ac.cn>
Reviewed-by: George Zhang <george.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 99f3af19073b3ddbfd96e789124cce12c4277b28)
Cc: stable@vger.kernel.org

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

@@ -12101,6 +12101,7 @@ static int dm_update_crtc_state(struct amdgpu_display_manager *dm,
 	/* Release extra reference */
 	if (new_stream)
 		dc_stream_release(new_stream);
+	new_stream = NULL;
 
 	/*
 	 * We want to do dc stream updates that do not require a