drm/i915/gt: Check set_default_submission() before deferencing

• cbl2 kernel 5.15.202.1-1 on CBL Mariner 2.0

• cbl2 kernel 5.15.202.1-1 on CBL-Mariner 2.0

SecurityAlert records field-level changes from the point we begin following a bulletin. Earlier vendor changes may not have a field-by-field record.

1. Oct 8, 2026 at 9:05 AM UTC
   • Added affected products: cbl2 kernel 5.15.202.1-1 on CBL-Mariner 2.0.
   • The vendor changed its remediation guidance.

1. Apr 30, 2026 at 1:10 AM UTC
   Published by Microsoft Security Response Center

   The publication date reported by the vendor.

2. Sep 25, 2026 at 2:09 AM UTC
   Updated by Microsoft Security Response Center

   The vendor changed the advisory after it was first published.

3. Sep 27, 2026 at 11:11 PM UTC
   Added to SecurityAlert

   We collected the advisory from the official source.

4. Nov 10, 2026 at 2:00 AM UTC
   Confirmed at the source

   Our collector saw this advisory during a later source check.