terminal/grdrm: force deep modeset on enter

Usually, when our session is activated (or re-configurated) we should be
able to try a page-flip to our buffer. The kernel driver should reject it
if it is incompatible. As it turns out, drivers don't do this. Therefore,
we now force a deep modeset if we're not sure what mode is set.

This has the side-effect that we might get glitches on session-switches
(depending on driver behavior). However, there's no way around this and it
is what everyone does so far. Most drivers still detect if we keep the
mode and so don't touch the clocks. Therefore, we just get a regular async
flip.

src/libsystemd-terminal/grdev-drm.c

@@ -1158,8 +1158,21 @@ static int grdrm_crtc_commit_flip(grdrm_crtc *crtc, grdev_fb *basefb) {
         assert(basefb);
         assert(pipe);
 
-        if (!crtc->applied && !grdrm_modes_compatible(&crtc->kern.mode, &crtc->set.mode))
+        if (!crtc->applied) {
+                if (!grdrm_modes_compatible(&crtc->kern.mode, &crtc->set.mode))
+                        return 0;
+
+                /* TODO: Theoretically, we should be able to page-flip to our
+                 * framebuffer here. We didn't perform any deep modeset, but the
+                 * DRM driver is really supposed to reject our page-flip in case
+                 * the FB is not compatible. We then properly fall back to a
+                 * deep modeset.
+                 * As it turns out, drivers don't to this. Therefore, we need to
+                 * perform a full modeset on enter now. We might avoid this in
+                 * the future with fixed drivers.. */
+
                 return 0;
+        }
 
         fb = fb_from_base(basefb);