drm/amdgpu: debugfs: fix NULL dereference in ta_if_invoke_debugfs_write()

If the kzalloc() fails then this code will crash.  Return -ENOMEM instead.

Fixes: e50d9ba0 ("drm/amdgpu: Add debugfs TA load/unload/invoke support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>