drm/i915/xehp: Use separate sseu init function

Xe_HP has enough fundamental differences from previous platforms that it
makes sense to use a separate SSEU init function to keep things
straightforward and easy to understand.  We'll also add a has_xehp_dss
flag to the SSEU structure that will be used by other upcoming changes.

v2:
 - Add has_xehp_dss flag

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220601150725.521468-2-matthew.d.roper@intel.com