drm/amdgpu: remove the redundant null checks

debugfs_remove and kfree has taken the null check in account.
hence it is unnecessary to check it. Just remove the condition.
No functional change.

This issue was detected by using the Coccinelle software.

Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>