drm/xe/buddy: remove the virtualized start

Hopefully not needed anymore. We can add a .compatible() hook once we
need to differentiate between mappable and non-mappable vram. If the
allocation is not contiguous then the start value is kind of
meaningless, so rather just mark as invalid.

In upstream, TTM wants to eventually remove the ttm_resource.start
usage.

References: 54443270

 ("drm/ttm: Add new callbacks to ttm res mgr")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>