drm/i915: Fix syncmap memory leak

A small race exists between intel_gt_retire_requests_timeout and
intel_timeline_exit which could result in the syncmap not getting
free'd. Rather than work to hard to seal this race, simply cleanup the
syncmap on fini.

unreferenced object 0xffff88813bc53b18 (size 96):
  comm "gem_close_race", pid 5410, jiffies 4294917818 (age 1105.600s)
  hex dump (first 32 bytes):
    01 00 00 00 00 00 00 00 00 00 00 00 0a 00 00 00  ................
    00 00 00 00 00 00 00 00 6b 6b 6b 6b 06 00 00 00  ........kkkk....
  backtrace:
    [<00000000120b863a>] __sync_alloc_leaf+0x1e/0x40 [i915]
    [<00000000042f6959>] __sync_set+0x1bb/0x240 [i915]
    [<0000000090f0e90f>] i915_request_await_dma_fence+0x1c7/0x400 [i915]
    [<0000000056a48219>] i915_request_await_object+0x222/0x360 [i915]
    [<00000000aaac4ee3>] i915_gem_do_execbuffer+0x1bd0/0x2250 [i915]
    [<000000003c9d830f>] i915_gem_execbuffer2_ioctl+0x405/0xce0 [i915]
    [<00000000fd7a8e68>] drm_ioctl_kernel+0xb0/0xf0 [drm]
    [<00000000e721ee87>] drm_ioctl+0x305/0x3c0 [drm]
    [<000000008b0d8986>] __x64_sys_ioctl+0x71/0xb0
    [<0000000076c362a4>] do_syscall_64+0x33/0x80
    [<00000000eb7a4831>] entry_SYSCALL_64_after_hwframe+0x44/0xa9

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Fixes: 531958f6

 ("drm/i915/gt: Track timeline activeness in enter/exit")
Cc: <stable@vger.kernel.org>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210730195342.110234-1-matthew.brost@intel.com