drm/i915: Fix some gcc warnings

Simple one:
drivers/gpu/drm/i915/i915_debugfs.c:2449:57: warning: Using plain integer as NULL pointer

And something a bit more peculiar:
drivers/gpu/drm/i915/i915_debugfs.c:4953:18: warning: Variable length array is used.
drivers/gpu/drm/i915/i915_debugfs.c:4953:32: warning: Variable length array is used.

We pass a 'const int' as the array size which results in the warning,
dropping the const gets rid of the warning. Weird, but I think getting
rid of the warnings is better than holding on to the const.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>