iris: Initialise stub iris_seqno to 0

iris: Initialise stub iris_seqno to 0

We create a stub never-signaled seqno to force the iris_fence to use the
fence fd, but we need to fully initialise the iris_seqno struct so that
the unset pointers are NULL and we do not try to destroy them later.

  ==38644== Conditional jump or move depends on uninitialised value(s)
  ==38644==    at 0xF7FBFAA: pipe_resource_reference (u_inlines.h:142)
  ==38644==    by 0xF7FC22F: iris_seqno_destroy (iris_seqno.c:38)
  ==38644==    by 0xF7E8930: iris_seqno_reference (iris_seqno.h:89)
  ==38644==    by 0xF7E8BC3: iris_fence_destroy (iris_fence.c:131)
  ==38644==    by 0xF7E8C41: iris_fence_reference (iris_fence.c:143)
  ==38644==    by 0xEF24525: dri2_destroy_fence (dri_helpers.c:176)
  ==38644==    by 0x4865DC2: dri2_egl_unref_sync (egl_dri2.c:3302)
  ==38644==    by 0x48661E8: dri2_destroy_sync (egl_dri2.c:3433)
  ==38644==    by 0x4855BA4: _eglDestroySync (eglapi.c:1952)
  ==38644==    by 0x4855CF5: eglDestroySyncKHR (eglapi.c:1972)
  ==38644==    by 0x402628: test_cleanup (egl_khr_fence_sync.c:232)
  ==38644==    by 0x40421E: test_eglCreateSyncKHR_native_from_fd (egl_khr_fence_sync.c:1521)

Closes: #2909
Fixes: fd1907efb385a6f66897 ("iris: Convert fences to using lightweight seqno")
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5004>