drm/xe: Move VRAM manager to struct xe_vram_region

VRAM manager is related directly to struct xe_vram_region so it
should be inside this structure.
Let's move the VRAM to struct xe_vram_region.

v2:
 - remove xe_vram_region pointer from xe_ttm_vram_mgr
 - stop use dynamic alloaction for xe_ttm_vram_mgr in xe_vram_region
 - rename struct xe_ttm_vram_mgr vram_mgr to ttm
v3:
 - fix "'ttm' not described in 'xe_vram_region'"

Signed-off-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250210081511.906452-3-piotr.piorkowski@intel.com