drm: avoid division by 0 in drm_pflip_cb with bad drivers

Seems like some drivers only increment msc every other page flip when
running in interlaced mode (I'm looking at you nouveau). I.e. it seems
to be incremented at the frame rate, rather than the field rate.
Obviously we can't work with this, so shame the driver and bail.

On intel this isn't an issue, as msc is incremented at field rate
there.

This means presentation feedback won't work correctly in interlaced
modes with those drivers, but who in their right mind uses an
interlaced mode these days, anyway?

video/out/drm_common.c

@@ -944,6 +944,15 @@ void drm_pflip_cb(int fd, unsigned int msc, unsigned int sec,
     const uint64_t ust = (sec * 1000000LL) + usec;
 
     const unsigned int msc_since_last_flip = msc - vsync->msc;
+    if (ready && msc == vsync->msc) {
+        // Seems like some drivers only increment msc every other page flip when
+        // running in interlaced mode (I'm looking at you nouveau). Obviously we
+        // can't work with this, so shame the driver and bail.
+        mp_err(closure->log,
+               "Got the same msc value twice: (msc: %u, vsync->msc: %u). This shouldn't happen. Possibly broken driver/interlaced mode?\n",
+               msc, vsync->msc);
+        goto fail;
+    }
 
     vsync->ust = ust;
     vsync->msc = msc;

video/out/drm_common.h

@@ -66,6 +66,7 @@ struct drm_pflip_cb_closure {
     struct drm_vsync_tuple *vsync; // vsync tuple of the latest page flip. drm_pflip_cb updates this
     struct vo_vsync_info *vsync_info; // where the drm_pflip_cb routine writes its output
     bool *waiting_for_flip; // drm_pflip_cb writes false here before returning
+    struct mp_log *log; // Needed to print error messages that shame bad drivers
 };
 
 bool vt_switcher_init(struct vt_switcher *s, struct mp_log *log);

video/out/opengl/context_drm_egl.c

@@ -459,6 +459,7 @@ static void queue_flip(struct ra_ctx *ctx, struct gbm_frame *frame)
     data->vsync = &p->vsync;
     data->vsync_info = &p->vsync_info;
     data->waiting_for_flip = &p->waiting_for_flip;
+    data->log = ctx->log;
 
     if (atomic_ctx) {
         drm_object_set_property(atomic_ctx->request, atomic_ctx->draw_plane, "FB_ID", p->fb->id);

video/out/vo_drm.c

@@ -489,6 +489,7 @@ static void queue_flip(struct vo *vo, struct kms_frame *frame)
     data->vsync = &p->vsync;
     data->vsync_info = &p->vsync_info;
     data->waiting_for_flip = &p->waiting_for_flip;
+    data->log = vo->log;
 
     ret = drmModePageFlip(p->kms->fd, p->kms->crtc_id,
                           p->cur_fb->fb,