recent radeondrm crashes should be fixed

[matthew green <mrg%eterna.com.au@localhost>]

hi folks.

i commited fixes that should fix the recent (since april 22 or
so) radeondrm crashes at X startup yesterday.

if anyone was having that problem, please update and try again.

the fixes also fix the radeondrm writeback test failure issue
that has been present since this codes day0 in netbsd.

thanks!


.mrg.