drm/xe: Use devm_ioremap_wc for VRAM mapping and drop manual unmap

drm/xe: Use devm_ioremap_wc for VRAM mapping and drop manual unmap

Let's replace the manual call to ioremap_wc function with devm_ioremap_wc
function, ensuring that VRAM mappings are automatically released when
the driver is detached.
Since devm_ioremap_wc registers the mapping with the device's managed
resources, the explicit iounmap call in vram_fini is no longer needed,
so let's remove it.

Signed-off-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Suggested-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Acked-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20250714184818.89201-2-piotr.piorkowski@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>