drm/panfrost: Prevent concurrent resets

If a job times out in slot 0 while a reset is performed because a job
timed out in slot 1, the drm-sched core can get into a deadlock.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190418084305.45021-1-tomeu.vizoso@collabora.com