dri3: Fix XACE access mode for open and get_supported_modifiers

Neither opening a screen nor querying its modifiers confers the right to
attach the buffer for any particular pixmap. GetAttr seems more correct.

Fixes: xorg/xserver#550
2 jobs for dri3-xace in 1 minute and 25 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Docker Image
passed debian-testing #82984

00:00:17

 
  Build And Test
passed build-and-test #82985

00:01:08