drm/i915: Restore memory mapping for DPT FBs across system suspend/resume

At least during hibernation the DPT mappings are lost with all stolen
memory content, so suspend/resume these mappings similarly to GGTT
mappings.

This fixes a problem where the restoring modeset during system resume fails
with pipe faults if a tiled framebuffer was active before suspend.

v2: Clarify the way restore works in intel_dpt_resume()'s Docbook entry.
    (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Vunny Sodhi <vunny.sodhi@intel.com>
Reported-and-tested-by: Vunny Sodhi <vunny.sodhi@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211101183551.3580546-2-imre.deak@intel.com