drm/xe/display: mark DPT with XE_BO_PAGETABLE

Otherwise in the case where we use normal system memory, the CPU access
will always be cached, like when filling the DPT PTEs, which is likely
not what we want since HW access could be incoherent on platforms like
LNL. Marking as XE_BO_PAGETABLE will force wc/uc underneath on such
platforms.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240314164905.239449-2-matthew.auld@intel.com