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.