drm/i915 Implement LMEM backup and restore for suspend / resume

drm/i915 Implement LMEM backup and restore for suspend / resume

Just evict unpinned objects to system. For pinned LMEM objects,
make a backup system object and blit the contents to that.

Backup is performed in three steps,
1: Opportunistically evict evictable objects using the gpu blitter.
2: After gt idle, evict evictable objects using the gpu blitter. This will
be modified in an upcoming patch to backup pinned objects that are not used
by the blitter itself.
3: Backup remaining pinned objects using memcpy.

Also move uC suspend to after 2) to make sure we have a functional GuC
during 2) if using GuC submission.

v2:
- Major refactor to make sure gem_exec_suspend@hang-SX subtests work, and
  suspend / resume works with a slightly modified GuC submission enabling
  patch series.

v3:
- Fix a potential use-after-free (Matthew Auld)
- Use i915_gem_object_create_shmem() instead of
  i915_gem_object_create_region (Matthew Auld)
- Minor simplifications (Matthew Auld)
- Fix up kerneldoc for i195_ttm_restore_region().
- Final lmem_suspend() call moved to i915_gem_backup_suspend from
  i915_gem_suspend_late, since the latter gets called at driver unload
  and we don't unnecessarily want to run it at that time.

v4:
- Interface change of ttm- & lmem suspend / resume functions to use
  flags rather than bools. (Matthew Auld)
- Completely drop the i915_gem_backup_suspend change (Matthew Auld)

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210922062527.865433-5-thomas.hellstrom@linux.intel.com