dma-fence: Simply wrap dma_fence_signal_locked with dma_fence_signal

Currently dma_fence_signal() tries to avoid the spinlock and only takes
it if absolutely required to walk the callback list. However, to allow
for some users to surreptitiously insert lazy signal callbacks that
do not depend on enabling the signaling mechanism around every fence,
we always need to notify the callbacks on signaling. As such, we will
always need to take the spinlock and dma_fence_signal() effectively
becomes a clone of dma_fence_signal_locked().

v2: Update the test_and_set_bit() before entering the spinlock.
v3: Drop the test_[and_set]_bit() before the spinlock, it's a caller
error so expected to be very unlikely.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Christian König <christian.koenig@amd.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190817152300.5370-1-chris@chris-wilson.co.uk