drm/i915/gt: Convert PSS_MODE2 to multicast register

That register became a multicast register as of Xe_HP and it is
currently used only for DG2. Use a proper prefix since there could be
usage of the same register for previous platforms in the future, which
would require a different definition (i.e. using _MMIO).

Note that, in its current state, the code does not cause functional
problems, since the actual application of the workaround would
implicitly use multicast mode. This fix is more toward consistency and
being future-proof uses of this register outside of workarounds.

v2:
  - Add paragraph noting that this change is for consistency and
    making the code future-proof. (Matt)

Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Matthew Atwood <matthew.s.atwood@intel.com>
Fixes: 468a4e63 ("drm/i915/dg2: Introduce Wa_18018764978")
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230120181423.90507-1-gustavo.sousa@intel.com