drm/i915/dg2: DG2 has fixed memory bandwidth

DG2 doesn't have a SAGV or QGV points that determine memory bandwidth.
Instead it has a constant amount of memory bandwidth available to
display that does not need to be reduced based on the number of active
planes.

For simplicity, we'll just modify driver initialization to create a
single dummy QGV point with the proper amount of memory bandwidth,
rather than trying to query the pcode for this information.

Bspec: 64631
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210721223043.834562-19-matthew.d.roper@intel.com