drm, drm/xe: Fix xe userptr in the absence of CONFIG_DEVICE_PRIVATE

drm, drm/xe: Fix xe userptr in the absence of CONFIG_DEVICE_PRIVATE

CONFIG_DEVICE_PRIVATE is not selected by default by some distros,
for example Fedora, and that leads to a regression in the xe driver
since userptr support gets compiled out.

It turns out that DRM_GPUSVM, which is needed for xe userptr support
compiles also without CONFIG_DEVICE_PRIVATE, but doesn't compile
without CONFIG_ZONE_DEVICE.
Exclude the drm_pagemap files from compilation with !CONFIG_ZONE_DEVICE,
and remove the CONFIG_DEVICE_PRIVATE dependency from CONFIG_DRM_GPUSVM and
the xe driver's selection of it, re-enabling xe userptr for those configs.

v2:
- Don't compile the drm_pagemap files unless CONFIG_ZONE_DEVICE is set.
- Adjust the drm_pagemap.h header accordingly.

Fixes: 9e9787414882 ("drm/xe/userptr: replace xe_hmm with gpusvm")
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: "Thomas Hellström" <thomas.hellstrom@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v6.18+
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patch.msgid.link/20260121091048.41371-2-thomas.hellstrom@linux.intel.com
(cherry picked from commit 1e372b246199ca7a35f930177fea91b557dac16e)
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>