drm/i915/gtt/dgfx: place the PD in LMEM

It's a requirement that for dgfx we place all the paging structures in
device local-memory.

v2: use i915_coherent_map_type()
v3: improve the shared dma-resv object comment

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210427085417.120246-4-matthew.auld@intel.com