drm/i915: Drop WARN_ON(flags) from ppgtt_bind_vma()

We will call ppgtt_bind_vma() with flags != 0, so the WARN_ON(flags)
is bogus. Kill it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>