drm/ast: Move cursor fb pinning and mapping into helper

The new helper ast_cursor_blit() updates a cursor's backbuffer HW
BO from a framebuffer structure. The cursor plane's prepare_fb()
function now uses the new interface.

Pinning and mapping of BOs is done automatically by the helper. This
includes the source BO, which was not pinned by the original code in
prepare_fb().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200702115029.5281-4-tzimmermann@suse.de