drm/xe: Do not sleep in atomic

Set atomic in xe_mmio_wait32() otherwise we would
be scheduling in atomic context.

Fixes: 7dc9b92d ("drm/xe: Remove i915_utils dependency from xe_pcode.")
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>