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
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.