drm/i915/gem: Return NULL instead of '0'

drm/i915/gem: Return NULL instead of '0'

Commit 05da7d9f717b ("drm/i915/gem: Downgrade stolen lmem setup
warning") returns '0' from i915_gem_stolen_lmem_setup(), but it's
supposed to return a pointer to the intel_memory_region
structure.

Sparse complains with the following message:

>> drivers/gpu/drm/i915/gem/i915_gem_stolen.c:943:32: sparse: sparse:
   Using plain integer as NULL pointer

Return NULL.

Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Jonathan Cavitt <jonathan.cavitt@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240617184243.330231-2-andi.shyti@linux.intel.com