drm/i915/psr: Fix race in intel_psr_work()

Commit 5422b37c907e ("drm/i915/psr: Kill delays when activating psr
back.") switched from delayed work to the plain variant and while doing so
removed the check for work_busy() before scheduling a PSR activation.
This appears to cause consecutive executions of psr_activate() in this
scenario - after a worker picks up the PSR work item for execution and
before the work function can acquire the PSR mutex, a psr_flush() can
get hold of the mutex and schedule another PSR work. Without a psr_exit()
between the two psr_activate() calls, warning messages get printed.
Further, since we drop the mutex in the midst of psr_work() to wait for
PSR to idle, another work item can also get scheduled. Fix this by
returning if PSR was already active.

Fixes: 5422b37c907e ("drm/i915/psr: Kill delays when activating psr back.")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106948
Cc: Rodrigo Vivi <[email protected]>
Cc: Chris Wilson <[email protected]>
Cc: José Roberto de Souza <[email protected]>
Signed-off-by: Dhinakaran Pandiyan <[email protected]>
Reviewed-by: Chris Wilson <[email protected]>
Reviewed-by: Rodrigo Vivi <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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