iommu/amd: Fix potential @entry null deref

After commit 26e495f3 ("iommu/amd: Restore IRTE.RemapEn bit after
programming IRTE"), smatch warns:

	drivers/iommu/amd/iommu.c:3870 amd_iommu_deactivate_guest_mode()
        warn: variable dereferenced before check 'entry' (see line 3867)

Fix this by moving the @valid assignment to after @entry has been checked
for NULL.

Fixes: 26e495f3 ("iommu/amd: Restore IRTE.RemapEn bit after programming IRTE")
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NJoao Martins <joao.m.martins@oracle.com>
Reviewed-by: NSuravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Cc: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Link: https://lore.kernel.org/r/20200910171621.12879-1-joao.m.martins@oracle.comSigned-off-by: NJoerg Roedel <jroedel@suse.de>

drivers/iommu/amd/iommu.c

@@ -3864,12 +3864,14 @@ int amd_iommu_deactivate_guest_mode(void *data)
 	struct amd_ir_data *ir_data = (struct amd_ir_data *)data;
 	struct irte_ga *entry = (struct irte_ga *) ir_data->entry;
 	struct irq_cfg *cfg = ir_data->cfg;
-	u64 valid = entry->lo.fields_remap.valid;
+	u64 valid;
 
 	if (!AMD_IOMMU_GUEST_IR_VAPIC(amd_iommu_guest_ir) ||
 	    !entry || !entry->lo.fields_vapic.guest_mode)
 		return 0;
 
+	valid = entry->lo.fields_remap.valid;
+
 	entry->lo.val = 0;
 	entry->hi.val = 0;