drm/xe: Rename struct xe_mem_region to struct xe_vram_region

The xe_mem_region structure has so far been used only in the context
of VRAM regions. Also, the description of its fields clearly indicates
that it was designed for VRAM regions. This struct is strictly related
only to VRAM.
So let's be clear on this point and rename it to xe_vram_region.

Signed-off-by: Piotr Piórkowski <piotr.piorkowski@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/20250210081511.906452-2-piotr.piorkowski@intel.com