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

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

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