Flush the correct number pf PTEs from cache after mapping a big framebuffer.
Essential for 4K displays. Thanks to Bakul Shah for finding this.

Reference: /n/sources/patch/bcm-mmukmap-bug
Date: Tue Jul  8 10:29:59 CES 2014
Signed-off-by: miller@hamnavoe.com

--- /sys/src/9/bcm/mmu.c	Tue Jul  8 10:28:13 2014
+++ /sys/src/9/bcm/mmu.c	Tue Jul  8 10:28:10 2014
@@ -304,7 +304,7 @@
 		*pte++ = (pa+n)|Dom0|L1AP(Krw)|Section;
 		mmuinvalidateaddr(va+n);
 	}
-	cachedwbse(pte0, pte - pte0);
+	cachedwbse(pte0, (uintptr)pte - (uintptr)pte0);
 	return va + o;
 }