drm/amd/display: remove redundant null pointer check before kfree

kfree has taken the null pointer into account. hence it is safe
to remove the redundant null pointer check before kfree.

Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>