glamor: don't fail create_pixmap for depth=30

xf86GetPixFormat has a hardcoded list of format, and there's no
30 bits format there.

But the format isn't needed to actually create the pixmap, so
make a NULL format a non-failure case.

This fixes an application trying to create a pbuffer using the first
config from glXChooseFBConfig(...GLX_PBUFFER_BIT...). This config happens
to be a 30 bits one and without this change the app would exit with
a BadAlloc error.
Status Name Job ID Coverage
  Docker Image
passed container-build #14971202

00:01:27

 
  Build
passed xserver-1.13 #14973618

00:00:24

passed xserver-1.14 #14973617

00:00:28

passed xserver-1.15 #14973616

00:00:30

passed xserver-1.16 #14973615

00:00:30

passed xserver-1.17 #14973614

00:00:30

passed xserver-1.18 #14973613

00:00:30

passed xserver-1.18-clang #14973612

00:00:34

passed xserver-1.19 #14973611

00:00:24

passed xserver-1.19-clang #14973591

00:00:32

passed xserver-1.20 #14973589

00:01:03

passed xserver-1.20-clang #14973590

00:00:32

failed xserver-1.13 #14971205

00:00:02

failed xserver-1.14 #14971206

00:00:03

failed xserver-1.15 #14971207

00:00:03

failed xserver-1.16 #14971208

00:00:02

failed xserver-1.17 #14971209

00:00:03

failed xserver-1.18 #14971210

00:00:02

failed xserver-1.18-clang #14971211

00:00:03

failed xserver-1.19 #14971212

00:00:03

failed xserver-1.19-clang #14971213

00:00:04

failed xserver-1.20 #14971203

00:00:02

failed xserver-1.20-clang #14971204

00:00:03