drm/amd/display: Fix warning comparing pointer to 0

Fix the following coccicheck warning:

./drivers/gpu/drm/amd/display/dc/clk_mgr/dcn301/vg_clk_mgr.c:744:35-36:
WARNING comparing pointer to 0.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>