drm/i915/xelpd: add XE_LPD display characteristics

Let's start preparing for upcoming platforms that will use an XE_LPD
design.

v2:
 - Use the now-preferred "XE_LPD" term to refer to this design
 - Utilize DISPLAY_VER() rather than a feature flag
 - Drop unused mbus_size field (Lucas)
v3:
 - Adjust for dbuf.{size,slice_mask} (Ville)

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com> (v2)
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210506161930.309688-2-imre.deak@intel.com