drm/i915: Reject opening of pipe crc files for invalid pipes

We don't init the lock nor set up all the other state. And it doesn't
make sense anyway.

This appeases lockdep when running the igt/drv_debugfs_reader test.

Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>