drm/amd/display: Remove redundant assignment to variable dc

Smatch report warning as follows:

Line 53679: drivers/gpu/drm/amd/display/dc/core/dc_stream.c:402
dc_stream_set_cursor_position() warn: variable dereferenced before
check 'stream'

The value of 'dc' has been assigned after check whether 'stream' is
NULL. Fix it by remove redundant assignment.
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Yi Yang <yiyang13@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_stream.c

@@ -408,7 +408,7 @@ bool dc_stream_set_cursor_position(
 	struct dc_stream_state *stream,
 	const struct dc_cursor_position *position)
 {
-	struct dc  *dc = stream->ctx->dc;
+	struct dc *dc;
 	bool reset_idle_optimizations = false;
 
 	if (NULL == stream) {