drm/radeon: fix uninitialized variable

Without this fix the driver randomly treats
textures as arrays and I'm really wondering
why gcc isn't complaining about it.

Signed-off-by: Christian König <deathsimple@vodafone.de>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>