drm/i915: Add DEBUG_GEM to the recommended CI config

Now that i915 compiles cleanly with Werror, we can enforce enabling
DEBUG_GEM when selecting the default debug config.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@intel.com>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210114113434.8229-1-chris@chris-wilson.co.uk