iris: fix in fences backend for ext_external_objects edge case

iris: fix in fences backend for ext_external_objects edge case

EXT_external_objects require we call glSignalSemaphoreEXT followed by a
glFlush. If the rendering workload is small when Signal and Flush
take place the relevant batch buffers with the actual rendering might
have been submitted already. In that case the following condition is met:
(iris_batch_bytes_used(batch) == 0). This causes:
glFlush() --> iris_fence_flush() -> iris_batch_flush() ->
_iris_batch_flush() to no-op, and so the fence doesn't get submitted to the
kernel. Then when anv tries to submit an execuf2 that must wait on the
shared VkSempahore / drm_syncobj fence, there isn't one and the kernel
rejects the batchbuffer causing an -EINVAL return of the execbuf2 ioctl
and a VK_DEVICE_LOST error. Empty batch buffers do have typically one
fence attached, but the ones carrying the extra fence from a
glSignalSempahore() call do have at least 2.

See also: the discussion in MR!4337.

v2: Changed the batch struct to have a contains_fence_signal variable
that is set to true when i915_EXEC_FENCE_SIGNAL fence is added to the
batch and off when batch is reset (Tapani Pälli)

Authored-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Reported-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Tested-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Eleni Maria Stea <elene.mst@gmail.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8861>