drm/amd/display: Fix memory leak in dcn21_clock_source_create

When dcn20_clk_src_construct() fails, we need to release clk_src.

Fixes: 6f4e6361

 ("drm/amd/display: Add Renoir resource (v2)")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>