drm/i915/gt: Correct prefix on a few registers

We have a few registers that have existed for several hardware
generations, but are only used by the driver on Xe_HP and beyond.  In
cases where the Xe_HP version of the register is now replicated and uses
multicast behavior, but earlier generations were singleton, let's change
the register prefix to "XEHP_" to help clarify that we're using the
newer multicast form of the register.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221014230239.1023689-5-matthew.d.roper@intel.com