iris: avoid stack overflow in iris_bo_wait_syncobj()

iris: avoid stack overflow in iris_bo_wait_syncobj()

Keep most cases using the stack as it's cheaper, but fall back to the
heap when the size gets too big.

This should fix a stack overflow reported by @rhezashan for a case
where we had lots of iris_screens.

Credits to Matt Turner and José Roberto de Souza for their work on
this issue, which led us to find its root cause.

Cc: mesa-stable
Reported-by: rheza shandikri (@rhezashan in gitlab)
Credits-to: José Roberto de Souza <jose.souza@intel.com>
Credits-to: Matt Turner <mattst88@gmail.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25236>