drm/i915/stolen: consider I915_BO_ALLOC_GPU_ONLY

Keep the behaviour consistent with normal lmem, where we assume CPU
access if by default required.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220315181425.576828-3-matthew.auld@intel.com