drm/i915/fdinfo: Don't show engine classes not present

Stop displaying engine classes with no engines - it is not a huge problem
if they are shown, since the values will correctly be all zeroes, but it
does count as misleading.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: 055634e4 ("drm/i915: Expose client engine utilisation via fdinfo")
Cc: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220616140056.559074-1-tvrtko.ursulin@linux.intel.com