drm/amdgpu: Remove a redundant sanity check

The case where 'num_entries' is too big, is already handled by
struct_size(), because kvmalloc() would fail.

It will return -ENOMEM instead of -EINVAL, but it is only related to a
unlikely to happen sanity check.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>