refactor: add helper function to check if VM is primary

The call `vm_is_primary(current->vm)` makes the intention more clear to
readers than `current->vm->id == HF_PRIMARY_VM_ID`.

Change-Id: I5cbf7293b8184da420ad6911cb810666ae9a45dc
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/src/ffa_memory.c b/src/ffa_memory.c
index f600d5e..028588d 100644
--- a/src/ffa_memory.c
+++ b/src/ffa_memory.c
@@ -969,7 +969,7 @@
 		 * are both valid representations of the !O-NA state.
 		 */
 		if (vm_id_is_current_world(to.vm->id) &&
-		    to.vm->id != HF_PRIMARY_VM_ID &&
+		    !vm_is_primary(to.vm) &&
 		    (orig_to_mode & MM_MODE_UNMAPPED_MASK) !=
 			    MM_MODE_UNMAPPED_MASK) {
 			return ffa_error(FFA_DENIED);