drm/xe/oa: Assign hwe for OAM_SAG

Because OAM_SAG doesn't have an attached hwe, assign another hwe belonging
to the same gt (and different OAM unit) to OAM_SAG. A hwe is needed for
batch submissions to program OA HW.

v2: Assign an engine with a valid OA unit for OAM_SAG (Umesh)

Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Link: https://lore.kernel.org/r/20250606192618.4133817-5-ashutosh.dixit@intel.com