Use xe_map_resource_to_region helper instead of direct access

Use xe_map_resource_to_region helper instead of direct access

Renaming:
The helper function res_to_mem_region is now xe_map_resource_to_region.

Abstraction:
The patch removes instances where block->private was accessed directly
to obtain VRAM region data allowing VRAM manager to own block->private
for future use cases.

V2(MattB): Add more detail about patch also adjust variable placement

Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20260407055107.2782450-2-tejas.upadhyay@intel.com
Signed-off-by: Tejas Upadhyay <tejas.upadhyay@intel.com>