drm/i915: Fix context runtime accounting

drm/i915: Fix context runtime accounting

When considering whether to mark one context as stopped and another as
started we need to look at whether the previous and new _contexts_ are
different and not just requests. Otherwise the software tracked context
start time was incorrectly updated to the most recent lite-restore time-
stamp, which was in some cases resulting in active time going backward,
until the context switch (typically the heartbeat pulse) would synchronise
with the hardware tracked context runtime. Easiest use case to observe
this behaviour was with a full screen clients with close to 100% engine
load.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: bb6287cb1886 ("drm/i915: Track context current active time")
Cc: <stable@vger.kernel.org> # v5.19+
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230320151423.1708436-1-tvrtko.ursulin@linux.intel.com
[tursulin: Fix spelling in commit msg.]