drm/i915: Correct typo in i915_vma_compress_finish stub

A copy and paste error in setting up the !CONFIG_DRM_I915_CAPTURE_ERROR
stubs left a conflicting duplicate declaration.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200111083007.1619228-1-chris@chris-wilson.co.uk