drm/i915: Fix oops due to missing stack depot

drm/i915: Fix oops due to missing stack depot

We call __save_depot_stack() unconditionally so the stack depot
must always be initialized or else we'll oops on platforms without
runtime pm support.

Presumably we've not seen this in CI due to stack_depot_init()
already getting called via drm_mm_init()+CONFIG_DRM_DEBUG_MM.

Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com> # stackdepot
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Imre Deak <imre.deak@intel.com>
Fixes: 2dba5eb1c73b ("lib/stackdepot: allow optional init and stack_table allocation by kvmalloc()")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220126081539.23227-1-ville.syrjala@linux.intel.com
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Imre Deak <imre.deak@intel.com>