drm/xe/oa: Introduce stream->oa_unit

Previously, the oa_unit associated with an OA stream was derived from hwe
associated with the stream (stream->hwe->oa_unit). This breaks with OAM_SAG
since OAM_SAG does not have any attached hardware engines. Resolve this by
introducing stream->oa_unit and stop depending on stream->hwe.

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/20250606192618.4133817-4-ashutosh.dixit@intel.com