drm/amdgpu: Fix deadlock on runtime suspend

amdgpu's ->runtime_suspend hook calls drm_kms_helper_poll_disable(),
which waits for the output poll worker to finish if it's running.

The output poll worker meanwhile calls pm_runtime_get_sync() in
amdgpu's ->detect hooks, which waits for the ongoing suspend to finish,
causing a deadlock.

Fix by not acquiring a runtime PM ref if the ->detect hooks are called
in the output poll worker's context.  This is safe because the poll
worker is only enabled while runtime active and we know that
->runtime_suspend waits for it to finish.

Fixes: d38ceaf9 ("drm/amdgpu: add core driver (v4)")
Cc: stable@vger.kernel.org # v4.2+: 27d4ee03: workqueue: Allow retrieval of current task's work struct
Cc: stable@vger.kernel.org # v4.2+: 25c058cc

: drm: Allow determining if current task is output poll worker
Cc: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Mike Lothian <mike@fireburn.co.uk>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Link: https://patchwork.freedesktop.org/patch/msgid/4c9bf72aacae1eef062bd134cd112e0770a7f121.1518338789.git.lukas@wunner.de