drm/i915/guc: Fix potential invalid pointer dereferences when decoding G2Hs

Some G2H handlers were reading the context id field from the payload
before checking the payload met the minimum length required.
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/20220302003357.4188363-9-John.C.Harrison@Intel.com

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

@@ -3895,12 +3895,13 @@ int intel_guc_deregister_done_process_msg(struct intel_guc *guc,
 					  u32 len)
 {
 	struct intel_context *ce;
-	u32 ctx_id = msg[0];
+	u32 ctx_id;
 
 	if (unlikely(len < 1)) {
 		drm_err(&guc_to_gt(guc)->i915->drm, "Invalid length %u\n", len);
 		return -EPROTO;
 	}
+	ctx_id = msg[0];
 
 	ce = g2h_context_lookup(guc, ctx_id);
 	if (unlikely(!ce))
@@ -3946,12 +3947,13 @@ int intel_guc_sched_done_process_msg(struct intel_guc *guc,
 {
 	struct intel_context *ce;
 	unsigned long flags;
-	u32 ctx_id = msg[0];
+	u32 ctx_id;
 
 	if (unlikely(len < 2)) {
 		drm_err(&guc_to_gt(guc)->i915->drm, "Invalid length %u\n", len);
 		return -EPROTO;
 	}
+	ctx_id = msg[0];
 
 	ce = g2h_context_lookup(guc, ctx_id);
 	if (unlikely(!ce))