vgabios: PMM scan was incorrectly depending on a zero %ds segment.

Make sure the PMM scanning code uses the GET_FARVAR macro.  (The
existing code only worked because SeaBIOS happens to call the vgabios
in bigreal mode with %ds == %ss = 0.)  Also, the scan doesn't require
bigreal mode - use accesses relative to the SEG_BIOS segment so that
the scan can work in regular real mode.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/vgasrc/vgainit.c b/vgasrc/vgainit.c
index 33b8eb9..6d9a224 100644
--- a/vgasrc/vgainit.c
+++ b/vgasrc/vgainit.c
@@ -60,14 +60,14 @@
 {
     if (!CONFIG_VGA_ALLOCATE_EXTRA_STACK)
         return;
-    void *pmmscan = (void*)BUILD_BIOS_ADDR;
-    for (; pmmscan < (void*)BUILD_BIOS_ADDR+BUILD_BIOS_SIZE; pmmscan+=16) {
-        struct pmmheader *pmm = pmmscan;
-        if (pmm->signature != PMM_SIGNATURE)
+    u32 pmmscan;
+    for (pmmscan=0; pmmscan < BUILD_BIOS_SIZE; pmmscan+=16) {
+        struct pmmheader *pmm = (void*)pmmscan;
+        if (GET_FARVAR(SEG_BIOS, pmm->signature) != PMM_SIGNATURE)
             continue;
-        if (checksum_far(0, pmm, pmm->length))
+        if (checksum_far(SEG_BIOS, pmm, GET_FARVAR(SEG_BIOS, pmm->length)))
             continue;
-        struct segoff_s entry = pmm->entry;
+        struct segoff_s entry = GET_FARVAR(SEG_BIOS, pmm->entry);
         dprintf(1, "Attempting to allocate VGA stack via pmm call to %04x:%04x\n"
                 , entry.seg, entry.offset);
         u16 res1, res2;