drm/i915/dpt: Make DPT object unshrinkable

In some scenarios, the DPT object gets shrunk but
the actual framebuffer did not and thus its still
there on the DPT's vm->bound_list. Then it tries to
rewrite the PTEs via a stale CPU mapping. This causes panic.

Cc: stable@vger.kernel.org
Reported-by: Shawn Lee <shawn.c.lee@intel.com>
Fixes: 0dc987b6 ("drm/i915/display: Add smem fallback allocation for dpt")
Signed-off-by: Vidya Srinivas <vidya.srinivas@intel.com>
[vsyrjala: Add TODO comment]
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240520165634.1162470-1-vidya.srinivas@intel.com