drm/i915/pmu: Add a name to the execlists stats

In preparation for GuC pmu stats, add a name to the execlists stats
structure so that it can be differentiated from the GuC stats.

Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211027004821.66097-1-umesh.nerlige.ramappa@intel.com