anv: Only wait for queue sync if execbuf was properly executed

anv: Only wait for queue sync if execbuf was properly executed

In case execbuf failed it would block execution until the maximum
timeout if DEBUG_SYNC is enabled.

While at it also removing the shadowing of result that would cause
the function result to not have its values updated in case
vk_sync_wait() returns a error.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18110>