drm/radeon/kms: don't read attempt to read bios from VRAM on unposted GPU.

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>