drm/xe: Move struct xe_vram_region to a dedicated header

drm/xe: Move struct xe_vram_region to a dedicated header

Let's move the xe_vram_region structure to a new header dedicated to VRAM
to improve modularity and avoid unnecessary dependencies when only
VRAM-related structures are needed.

v2: Fix build if CONFIG_DRM_XE_DEVMEM_MIRROR is enabled
v3: Fix build if CONFIG_DRM_XE_DISPLAY is enabled
v4: Move helper to get tile dpagemap to xe_svm.c

Signed-off-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Suggested-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Satyanarayana K V P <satyanarayana.k.v.p@intel.com> # rev3
Acked-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250714184818.89201-4-piotr.piorkowski@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>