All timestamps returned by GuC for GuC PMU busyness are captured from
GUC PM TIMESTAMP. Since this timestamp does not tick when GuC goes idle,
kmd uses RING_TIMESTAMP to measure busyness of an engine with an active
context. In further stress testing, the MMIO read of the RING_TIMESTAMP
is seen to cause a rare hang. Resolve the issue by using gt specific
timestamp from PM which is in sync with the GuC PM timestamp.

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>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220111015523.225562-1-umesh.nerlige.ramappa@intel.com