drm/i915: Filter out spurious execlists context-switch interrupts

Back in commit a4b2b01523a8 ("drm/i915: Don't mark an execlists
context-switch when idle") we noticed the presence of late
context-switch interrupts. We were able to filter those out by looking
at whether the ELSP remained active, but in commit beecec901790
("drm/i915/execlists: Preemption!") that became problematic as we now
anticipate receiving a context-switch event for preemption while ELSP
may be empty. To restore the spurious interrupt suppression, add a
counter for the expected number of pending context-switches and skip if
we do not need to handle this interrupt to make forward progress.

v2: Don't forget to switch on for preempt.
v3: Reduce the counter to a on/off boolean tracker. Declare the HW as
active when we first submit, and idle after the final completion event
(with which we confirm the HW says it is idle), and track each source
of activity separately. With a finite number of sources, it should aide
us in debugging which gets stuck.

Fixes: beecec901790 ("drm/i915/execlists: Preemption!")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Michal Winiarski <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: Arkadiusz Hiler <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Mika Kuoppala <[email protected]>
(cherry picked from commit 4a118ecbe99c93cf9f9582e83a88d03f18d6cb84)
Signed-off-by: Jani Nikula <[email protected]>

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