drm/nv50-/kms: reject attempts at flipping to incompatible framebuffer

Looks like a userspace bug can trigger this somehow during a mode
switch, causing: EVO complaint -> semaphores get out of sync ->
entire display stalled.

We likely want to be even stricter than this (or at least deal
better if EVO rejects our request), but I'll save that for the
drm_plane/atomic conversion and just fix the bug that I already
know can be triggered.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nv50_display.c

@@ -550,6 +550,10 @@ nv50_display_flip_next(struct drm_crtc *crtc, struct drm_framebuffer *fb,
 	u32 *push;
 	int ret;
 
+	if (crtc->primary->fb->width != fb->width ||
+	    crtc->primary->fb->height != fb->height)
+		return -EINVAL;
+
 	swap_interval <<= 4;
 	if (swap_interval == 0)
 		swap_interval |= 0x100;