drm/i915: Update object placement flags to be mutable

The object ops i915_GEM_OBJECT_HAS_IOMEM and the object
I915_BO_ALLOC_STRUCT_PAGE flags are considered immutable by
much of our code. Introduce a new mem_flags member to hold these
and make sure checks for these flags being set are either done
under the object lock or with pages properly pinned. The flags
will change during migration under the object lock.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210624084240.270219-2-thomas.hellstrom@linux.intel.com