drm/i915: memory leak in __i915_gem_vma_create()

In the original code then if WARN_ON(i915_is_ggtt(vm) != !!ggtt_view)
was true then we leak "vma".  Presumably that doesn't happen often but
static checkers complain and this bug is easy to fix.

Fixes: c3bbb6f2825d ('drm/i915: Do not use ggtt_view with (aliasing) PPGTT')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>