drm/radeon/kms: fix potential segfault in r600_ioctl_wait_idle

radeon_gem_wait_idle_ioctl can apparently get called prior to
the vram page being set up or even if accel if false, so make
sure it's valid before using it.

Should fix:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=597636
https://bugs.freedesktop.org/show_bug.cgi?id=29834

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>