glamor: use gbm_format_for_depth, x11/xwayland: enable 30-bit pixmaps

The goal is to fix piglit glx-visuals-depth -pixmap and glx-visuals-stencil -pixmap tests.

The first commit is required to support all possible depth formats received.

The second commit, is to fix this error:

                    glXGetFBConfigAttrib(dpy, config, GLX_FBCONFIG_ID, &id);
                    fprintf(stderr, "fbconfig %d has GLX_PIXMAP_BIT but there "
                                    "is no pixmap format for depth %d, this "
                                    "is a server bug\n", id, depth);

that appears when XListDepths and glXGetFBConfigs disagrees on 30-bit support.