drm/i915/guc: Fix a static analysis warning

A static analyser was complaining about not checking for null
pointers. However, the location of the complaint can only be reached
in the first place if said pointer is non-null. Basically, if we are
using a v69 GuC then the descriptor pool is guaranteed to be alocated
at start of day or submission will be disabled with an ENOMEM error.
And if we are using a later GuC that does not use a descriptor pool
then the v69 submission function would not be called. So, not a
possible null at that point in the code.

Hence adding a GEM_BUG_ON(!ptr) to keep the tool happy.
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221221193031.687266-3-John.C.Harrison@Intel.com

drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c

@@ -2533,6 +2533,7 @@ static void prepare_context_registration_info_v69(struct intel_context *ce)
 		   i915_gem_object_is_lmem(ce->ring->vma->obj));
 
 	desc = __get_lrc_desc_v69(guc, ctx_id);
+	GEM_BUG_ON(!desc);
 	desc->engine_class = engine_class_to_guc_class(engine->class);
 	desc->engine_submit_mask = engine->logical_mask;
 	desc->hw_context_desc = ce->lrc.lrca;