drm/xe: Add warn when level can not be zero.

drm/xe: Add warn when level can not be zero.

At xe_pt_zap_ptes_entry() and xe_pt_stage_unbind_entry, the level cannot
be 0. Therefore, add an independent check for the level. Since the level
cannot be zero at this point, there is no need to check for `is_compact`,
so remove that instead.

Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240521103623.11645-1-nirmoy.das@intel.com
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>