i915/pmu: Move execlist stats initialization to execlist specific setup

engine->stats is a union of execlist and guc stat objects. When execlist
specific fields are initialized, the initial state of guc stats is
affected. This results in bad busyness values when using GuC mode. Move
the execlist initialization from common code to execlist specific code.

Fixes: 77cdd054 ("drm/i915/pmu: Connect engine busyness stats from GuC to pmu")
Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Alan Previn <alan.previn.teres.alexis@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230912212247.1828681-1-umesh.nerlige.ramappa@intel.com