drm/xe: Introduce CONFIG_DRM_XE_GPUSVM

Don't rely on CONFIG_DRM_GPUSVM because other drivers may enable it
causing us to compile in SVM support unintentionally.

Also take the opportunity to leave more code out of compilation if
!CONFIG_DRM_XE_GPUSVM and !CONFIG_DRM_XE_DEVMEM_MIRROR

v3:
- Fixes for compilation errors on 32-bit. This changes the Kconfig
  logic a bit.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250326080551.40201-2-thomas.hellstrom@linux.intel.com