drm/xe: Make usable size of VRAM readable

Current size member of vram struct does not give
complete information as what "size" contains. Does
it contain reserved portions or not. Name it usable
size and accordingly describe other size members as
well.

Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>