drm/i915: Silence compiler for GTT selftests

gcc-4.7 spotted that

In file included from drivers/gpu/drm/i915/i915_gem_gtt.c:3791:0:
drivers/gpu/drm/i915/selftests/i915_gem_gtt.c: In function ‘pot_hole’:
drivers/gpu/drm/i915/selftests/i915_gem_gtt.c:594:6: error: ‘err’ may be used uninitialized in this function [-Werror=maybe-uninitialized]

So set it to 0 should we ever skip over a hole smaller than a few pages.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170214113756.27834-1-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>