drm/mgag200: Don't unpin the current cursor image's buffer.

Currently the displayed cursor buffer might be evicted from video memory.
Not unpinning the BO fixes this problem. At this point, pixels_current
also references the BO and it will be unpinned during the next cursor
update.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Fixes: 94dc57b1 ("drm/mgag200: Rewrite cursor handling")
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190723075425.24028-4-tzimmermann@suse.de

drivers/gpu/drm/mgag200/mgag200_cursor.c

@@ -213,7 +213,6 @@ int mga_crtc_cursor_set(struct drm_crtc *crtc,
 	mdev->cursor.pixels_current = pixels_next;
 
 	drm_gem_vram_kunmap(pixels_next);
-	drm_gem_vram_unpin(pixels_next);
 	drm_gem_vram_kunmap(gbo);
 	drm_gem_vram_unpin(gbo);
 	drm_gem_object_put_unlocked(obj);