drm/gpu: Remove dead checks on wbinvd_on_all_cpus()'s return value

Remove the checks and associated pr_err() on wbinvd_on_all_cpus() failure,
as the helper has unconditionally returned 0/success since commit

  caa75932 ("smp: Remove smp_call_function() and on_each_cpu() return values").

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250522233733.3176144-2-seanjc@google.com