iris: add support for fence signal capability

iris: add support for fence signal capability

This enables GL_EXT_semaphore feature.

v2:
* reversed previous commit that was conditionally setting the signal
  fence capability if the syncobj was present
* reversed previous commit that was introducing a bool has_syncobj that
  is not necessary anymore

v3:
* changed the signal function to use fence->seqno due to recent changes
  to master

v4:
* changed the signal callback to use the new structs of the fences
  backend (iris_fine_fence)

v5:
* removed check for ctx == NULL in iris_fence_signal and await functions
  as at the time they are called we always have a context
* splitted a line to not exceed width

v6:
* put back the if(ctx) check in iris_fence_await, if this is an error
  the fix should be in a different MR

Signed-off-by: Eleni Maria Stea <estea@igalia.com>
Reviewed-by: Rohan Garg <rohan.garg@collabora.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7042>