glamor: xv: Fix invalid accessing of plane attributes for NV12

NV12 only has 2 planes.

Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
3 jobs for nv12 in 6 minutes and 6 seconds (queued for 1 second)
latest
Status Name Job ID Coverage
  Docker Image
passed debian-testing #908926

00:00:38

 
  Build And Test
passed autotools-build-and-test #908927

00:05:27

passed meson-build-and-test #908928

00:01:30