render: don't open DRM node if WLR_RENDERER=pixman
Opening a DRM node is a waste of time if the user already explicitly specified pixman, and it also trips Gentoo's build sandbox.
The logic is somewhat duplicated between wlr_renderer_autocreate and renderer_autocreate_with_drm_fd, but it's hard to fix easily because init_drm_renderer calls renderer_autocreate_with_drm_fd directly.