dma-fence: Add a single fence fast path for fence merging

Testing some workloads in two different scenarios, such as games running
under Gamescope on a Steam Deck, or vkcube under a Plasma desktop, shows
that in a significant portion of calls the dma_fence_unwrap_merge helper
is called with just a single unsignalled fence.

Therefore it is worthile to add a fast path for that case and so bypass
the memory allocation and insertion sort attempts.

Tested scenarios:

1) Hogwarts Legacy under Gamescope

~1500 calls per second to __dma_fence_unwrap_merge.

Percentages per number of fences buckets, before and after checking for
signalled status, sorting and flattening:

   N       Before      After
   0       0.85%
   1      69.80%        ->   The new fast path.
  2-9     29.36%        9%   (Ie. 91% of this bucket flattened to 1 fence)
 10-19
 20-40
  50+

2) Cyberpunk 2077 under Gamescope

~2400 calls per second.

   N       Before      After
   0       0.71%
   1      52.53%        ->    The new fast path.
  2-9     44.38%      50.60%  (Ie. half resolved to a single fence)
 10-19     2.34%
 20-40     0.06%
  50+

3) vkcube under Plasma

90 calls per second.

   N       Before      After
   0
   1
  2-9      100%         0%   (Ie. all resolved to a single fence)
 10-19
 20-40
  50+

In the case of vkcube all invocations in the 2-9 bucket were actually
just two input fences.

v2:
 * Correct local variable name and hold on to unsignaled reference. (Chistian)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Friedrich Vock <friedrich.vock@gmx.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241115102153.1980-4-tursulin@igalia.com

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