drm/i915: make pool objects read-only

For our current users we don't expect pool objects to be writable from
the gpu.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Fixes: 4f7af194 ("drm/i915: Support ro ppgtt mapped cmdparser shadow buffers")
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191119150154.18249-1-matthew.auld@intel.com