drm/xe/xe_pmu: Add PMU support for per-function engine activity stats

drm/xe/xe_pmu: Add PMU support for per-function engine activity stats

Add PMU support for per-function engine activity stats.

per-function engine activity is enabled when VF's are enabled.
If 2 VF's are enabled, then the applicable function values are

0 - PF engine activity
1 - VF1 engine activity
2 - VF2 engine activity

This can be read from perf tool as shown below

./perf stat -e xe_<bdf>/engine-active-ticks,gt=0,engine_class=0,
			 engine_instance=0,function=1/ -I 1000

v2: fix documentation (Umesh)
    remove global for functions (Lucas, Michal)

v3: fix commit message
    move function_id checks to same place (Michal)

v4: fix comment (Umesh)

Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Riana Tauro <riana.tauro@intel.com>
Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250311071759.2117211-3-riana.tauro@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>