drm/i915/display: handle migration for dpt

On platforms like DG2, it looks like the dpt path here is missing the
migrate-to-lmem step on discrete platforms.

v2:
  - Move the vma_pin() under the for_i915_gem_ww(), otherwise the
    object can be moved after dropping the lock and then doing the pin.

Fixes: 33e7a975

 ("drm/i915/xelpd: First stab at DPT support")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Jianshui Yu <jianshui.yu@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221004131916.233474-2-matthew.auld@intel.com