Add workaround for vt efifb's early use of PHYS_TO_DMAP

In vt_efifb_init the framebuffer's physaddr is passed to PHYS_TO_DMAP
before the DMAP is setup. The result is not actually accessed until
after the mapping is setup, though. Loosen the assertion in PHYS_TO_DMAP
for now, to allow use when dmaplimit == 0.

Reviewed by:	kib
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D1142

sys/amd64/include/vmparam.h

@@ -175,8 +175,14 @@
 #define	VM_MAX_ADDRESS		UPT_MAX_ADDRESS
 #define	VM_MIN_ADDRESS		(0)
 
+/*
+ * XXX Allowing dmaplimit == 0 is a temporary workaround for vt(4) efifb's
+ * early use of PHYS_TO_DMAP before the mapping is actually setup. This works
+ * because the result is not actually accessed until later, but the early
+ * vt fb startup needs to be reworked.
+ */
 #define	PHYS_TO_DMAP(x)	({						\
-	KASSERT((x) < dmaplimit,					\
+	KASSERT(dmaplimit == 0 || (x) < dmaplimit,			\
 	    ("physical address %#jx not covered by the DMAP",		\
 	    (uintmax_t)x));						\
 	(x) | DMAP_MIN_ADDRESS; })