drm/i915: Track all held rpm wakerefs

Everytime we take a wakeref, record the stack trace of where it was
taken; clearing the set if we ever drop back to no owners. For debugging
a rpm leak, we can look at all the current wakerefs and check if they
have a matching rpm_put.

v2: Use skip=0 for unwinding the stack as it appears our noinline
function doesn't appear on the stack (nor does save_stack_trace itself!)
v3: Allow rpm->debug_count to disappear between inspections and so
avoid calling krealloc(0) as that may return a ZERO_PTR not NULL! (Mika)
v4: Show who last acquire/released the runtime pm

Signed-off-by: Chris Wilson <[email protected]>
Cc: Jani Nikula <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Reviewed-by: Mika Kuoppala <[email protected]>
Tested-by: Mika Kuoppala <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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