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devvga: better way to prevent blank hwaccel race

don't hold drawlock duing vga enable and disable, but just zero
the function pointers under drawlock *before* disabling the vga
device.

holding the drawlock while calling out into enable and disable
is not a good idea. with vgavesa, this might deadlock when
userspace realemu tries to print in a rio window with vgavesa.

--- a/sys/src/9/pc/devvga.c

+++ b/sys/src/9/pc/devvga.c

@@ -309,23 +309,18 @@

 		for(i = 0; vgadev[i]; i++){

 			if(strcmp(cb->f[1], vgadev[i]->name))

 				continue;

-			qlock(&drawlock);

-			if(waserror()){

-				qunlock(&drawlock);

-				nexterror();

-			}

 			if(scr->dev){

-				if(scr->dev->disable)

-					scr->dev->disable(scr);

+				qlock(&drawlock);

 				scr->fill = nil;

 				scr->scroll = nil;

 				scr->blank = nil;

+				qunlock(&drawlock);

+				if(scr->dev->disable)

+					scr->dev->disable(scr);

 			}

 			scr->dev = vgadev[i];

 			if(scr->dev->enable)

 				scr->dev->enable(scr);

-			qunlock(&drawlock);

-			poperror();

 			return;

 		}

 		break;