intel_gpu_top: Improve error message when insufficient privilege

Print out end user friendly help text when the running user has
insufficient privilege for accessing system wide performance counters.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Issue: https://gitlab.freedesktop.org/drm/intel/-/issues/5018


Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>