drm/i915: fix uninitialized variable warning in i915_setup_compression()

Fixes:

drivers/gpu/drm/i915/i915_dma.c: In function ‘i915_setup_compression’:
drivers/gpu/drm/i915/i915_dma.c:1311: error: ‘compressed_llb’ may be used uninitialized in this function

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Eric Anholt <eric@anholt.net>