drm/i915: Allow NULL memory region

Prevent a NULL pointer access in intel_memory_regions_hw_probe.

Fixes: 05da7d9f ("drm/i915/gem: Downgrade stolen lmem setup warning")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/11704Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240712214156.3969584-1-jonathan.cavitt@intel.com
(cherry picked from commit d75dec1f)
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>

drivers/gpu/drm/i915/intel_memory_region.c

@@ -368,8 +368,10 @@ int intel_memory_regions_hw_probe(struct drm_i915_private *i915)
 			goto out_cleanup;
 		}
 
-		mem->id = i;
-		i915->mm.regions[i] = mem;
+		if (mem) { /* Skip on non-fatal errors */
+			mem->id = i;
+			i915->mm.regions[i] = mem;
+		}
 	}
 
 	for (i = 0; i < ARRAY_SIZE(i915->mm.regions); i++) {