drm/i915: Return NULL on error in active_instance

drm/i915: Return NULL on error in active_instance

Avoid returning &node->base when node is NULL due to OOM
during GFP_ATOMIC allocation.

Discovered using AI-assisted static analysis confirmed by
Intel Product Security.

Reported-by: Martin Hodo <martin.hodo@intel.com>
Fixes: bfaae47db3c0 ("drm/i915: make lockdep slightly happier about execbuf.")
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Simona Vetter <simona.vetter@ffwll.ch>
Cc: <stable@vger.kernel.org> # v5.13+
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Sebastian Brzezinka <sebastian.brzezinka@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patch.msgid.link/20260624090940.74840-1-joonas.lahtinen@linux.intel.com
(cherry picked from commit 6029bc064f0b1bac184203a50fbaaf070fa18832)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>