Since the VGA switcheroo, we'd attempt to read the BIOS from VRAM on startup
but on some unposted cards this can cause hangs/crashes.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=28592

(further problem pointed out by agd5f on IGP systems)

Reported-by: Reilithion on #radeon
Signed-off-by: Dave Airlie <airlied@redhat.com>