drm/xe/mmio: Add check for minimal BAR size

drm/xe/mmio: Add check for minimal BAR size

We initialized the root tile's xe_mmio structure with a new size
of 4MiB without sanity checks to see if mapped GTTMMADR_BAR was
actually at least that size. Check BAR size against first 16MiB,
which is expected minimum BAR size for the one-tile platforms.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patch.msgid.link/20260622132342.19600-4-michal.wajdeczko@intel.com