Do it uncontionally, there's a separate peek function with
dma_fence_is_signalled() which can be called from atomic context.

v2: Consensus calls for an unconditional might_sleep (Chris,
Christian)

Full audit:
- dma-fence.h: Uses MAX_SCHEDULE_TIMOUT, good chance this sleeps
- dma-resv.c: Timeout always at least 1
- st-dma-fence.c: Save to sleep in testcases
- amdgpu_cs.c: Both callers are for variants of the wait ioctl
- amdgpu_device.c: Two callers in vram recover code, both right next
  to mutex_lock.
- amdgpu_vm.c: Use in the vm_wait ioctl, next to _reserve/unreserve
- remaining functions in amdgpu: All for test_ib implementations for
  various engines, caller for that looks all safe (debugfs, driver
  load, reset)
- etnaviv: another wait ioctl
- habanalabs: another wait ioctl
- nouveau_fence.c: hardcoded 15*HZ ... glorious
- nouveau_gem.c: hardcoded 2*HZ ... so not even super consistent, but
  this one does have a WARN_ON :-/ At least this one is only a
  fallback path for when kmalloc fails. Maybe this should be put onto
  some worker list instead, instead of a work per unamp ...
- i915/selftests: Hardecoded HZ / 4 or HZ / 8
- i915/gt/selftests: Going up the callchain looks safe looking at
  nearby callers
- i915/gt/intel_gt_requests.c. Wrapped in a mutex_lock
- i915/gem_i915_gem_wait.c: The i915-version which is called instead
  for i915 fences already has a might_sleep() annotation, so all good

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: "VMware Graphics" <linux-graphics-maintainer@vmware.com>
Cc: Oded Gabbay <oded.gabbay@gmail.com>
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Cc: linux-rdma@vger.kernel.org
Cc: amd-gfx@lists.freedesktop.org
Cc: intel-gfx@lists.freedesktop.org
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Christian König <christian.koenig@amd.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200519132756.682888-1-daniel.vetter@ffwll.ch