drm/i915: Prefer "XEHP_" prefix for registers

We've been introducing new registers with a mix of "XEHP_"
(architecture) and "XEHPSDV_" (platform) prefixes.  For consistency,
let's settle on "XEHP_" as the preferred form.

XEHPSDV_RP_STATE_CAP stays with its current name since that's truly a
platform-specific register and not something that applies to the Xe_HP
architecture as a whole.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Caz Yokoyama <caz@caztech.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220624210328.308630-2-matthew.d.roper@intel.com