kms: Add way to run without mode setting

Currently our only entry point for DRM devices is MetaKms*, but in order
to run without being DRM master, we cannot use /dev/dri/card*, nor can
we be either of the existing MetaKmsImplDevice implementation (legacy
KMS, and atomic KMS), as they both depend on being DRM master.

Thus to handle running without being DRM master (i.e. headless), add a
"dummy" MetaKmsImplDevice implementation, that doesn't do any mode
setting at all, and that switches to operate on the render node, instead
of the card node itself.

This means we still use the same GBM code paths as the regular native
backend paths, except we never make use of any CRTC backed onscreen
framebuffers.

Eventually, this "dummy" MetaKmsImplDevice will be replaced separating
"KMS" device objects from "render" device objects, but that will require
more significant changes. It will, however, be necessary for e.g. going
from being headless, only having access to a render node, to turning
into a real session, with a seat, being DRM master, and having access to
a card node.

This is currently not hooked up, but will be in a later commit.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1698>

src/backends/native/meta-backend-native.c

@@ -531,7 +531,9 @@ meta_backend_native_initable_init (GInitable     *initable,
 
   native->udev = meta_udev_new (native);
 
-  native->kms = meta_kms_new (META_BACKEND (native), error);
+  native->kms = meta_kms_new (META_BACKEND (native),
+                              META_KMS_FLAG_NONE,
+                              error);
   if (!native->kms)
     return FALSE;