drm/i915: Remove assertion from raw __i915_vma_unpin()

drm/i915: Remove assertion from raw __i915_vma_unpin()

After we detect a i915_vma pin overflow, we call __i915_vma_unpin to
cleanup. However, on an overflow the pin_count bitfield will be zero,
triggering an assertion, even though we the intention is to merely warn
and report the error back to the user (as historically the culprit has
be a leak in the display code).

Fixes: 20dfbde463c8 ("drm/i915: Wrap vma->pin_count accessors with small inline helpers")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170721145037.25105-2-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>