drm/i915: fix unused var

drivers/gpu/drm/i915/i915_dma.c: In function 'i915_driver_load':
drivers/gpu/drm/i915/i915_dma.c:1114: warning: 'll_base' may be used uninitialized in this function

Partly this is because gcc isn't smart enough.  But `ll_base' does get used
uninitialised in the DRM_DEBUG() call.

Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Eric Anholt <eric@anholt.net>
Cc: Dave Airlie <airlied@linux.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Eric Anholt <eric@anholt.net>