After vga framebuffer is set to write combining mode, set mmio control register page
(which is inside framebuffer space) back to uncached.

Reference: /n/sources/patch/applied/vgamach64xx-mtrr-fix
Date: Tue Jun 14 12:06:34 CES 2011
Signed-off-by: miller@hamnavoe.com

--- /sys/src/9/pc/vgamach64xx.c	Tue Jun 14 12:01:10 2011
+++ /sys/src/9/pc/vgamach64xx.c	Tue Jun 14 12:01:07 2011
@@ -193,6 +193,15 @@
 	vgalinearpci(scr);
 	if(scr->paddr == 0)
 		return;
+	/*
+	 * vgalinearpci sets framebuffer into write combining mode.
+	 * Because mmio register page is inside framebuffer space,
+	 * set it back to uncached.
+	 */
+	if(!waserror()){
+		mtrr(scr->paddr+size-BY2PG, BY2PG, "uc");
+		poperror();
+	}
 	scr->mmio = (ulong*)((uchar*)scr->vaddr+size-1024);
 	addvgaseg("mach64mmio", scr->paddr+size-BY2PG, BY2PG);
 	addvgaseg("mach64screen", scr->paddr, scr->apsize);