drm/i915: Make i915_gem_object_phys_attach() use obj->mm.lock more appropriately

Actually transferring from shmemfs to the physically contiguous set of
pages should be wholly guarded by its obj->mm.lock!

v2: Remember to free the old pages.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170726160038.29487-2-chris@chris-wilson.co.uk


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>