drm/xe: enforce GSMBASE for DG1 instead of BAR2

On DG1, BAR2 is not reliable for reporting Vram size, need to use GSMBASE.
Simplify xe_mmio_total_vram_size to report vram size and usable size.

Signed-off-by: Philippe Lecluse <philippe.lecluse@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>