drm/xe: Introduce xe_engine_is_idle()

drm/xe: Introduce xe_engine_is_idle()

Introduce xe_engine_is_idle, and replace the static function in
xe_migrate.c.
The latter had two flaws. First the seqno == 1 test might return a false
true value each time the seqno counter wrapped, Second, the
cur_seqno == next_seqno test would never return true.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>