drm/i915/dsb: fix extra warning on error path handling

When we call intel_dsb_get(), the dsb initialization may fail for
various reasons. We already log the error message in that path, making
it unnecessary to trigger a warning that refcount == 0 when calling
intel_dsb_put().

So here we simplify the logic and do lazy shutdown: leaving the extra
refcount alive so when we call intel_dsb_put() we end up calling
i915_vma_unpin_and_release().

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191111205024.22853-3-lucas.demarchi@intel.com