drm/i915: make sparse happy for the new vlv mmio read function

It doesn't like that we assign 0 to a pointer, it wants the real NULL.

On closer look that initialization is actually bogus, and the compiler
can easily see that we never use it unitialized. So let's just drop
this.

Cc: Deepak S <deepak.s@intel.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>