drm/xe/pxp: Initialize PXP structure and KCR reg

As the first step towards adding PXP support, hook in the PXP init
function, allocate the PXP structure and initialize the KCR register to
allow PXP HWDRM sessions.

v2: remove unneeded includes, free PXP memory on error (John)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250129174140.948829-2-daniele.ceraolospurio@intel.com