iris: Store iris_context's priority

iris: Store iris_context's priority

This way when replacing a broken context we don't need to ask to
kernel what is the priority of the context being replaced.

Also this will be necessary for Xe kmd as it don't have any uapi to
query engine priority.

While doing that also taking the oportunity to move more code from
iris_bufmgr.c/h that only has one caller.

Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21965>