drm/i915: Don't deref context->file_priv ERR_PTR upon reset

When a user context is closed, it's file_priv backpointer is replaced by
ERR_PTR(-EBADF); be careful not to chase this invalid pointer after a
hang and a GPU reset.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Fixes: b083a087 ("drm/i915: Add per client max context ban limit")
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161122144121.7379-1-chris@chris-wilson.co.uk