drm/ast: Don't enable HW cursors twice during atomic update

The ast_cursor_show() helper enables the cursor to be displayed. No need
to repeat that operation in the plane's atomic-update function.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200702115029.5281-8-tzimmermann@suse.de