drm/i915: Allow NULL memory region

drm/i915: Allow NULL memory region

Prevent a NULL pointer access in intel_memory_regions_hw_probe.

Fixes: 05da7d9f717b ("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/11704
Signed-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