drm/ast: Don't pin cursor source BO explicitly during update

Vmapping the cursor source BO contains an implicit pin operation,
so there's no need to do this manually.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201209142527.26415-2-tzimmermann@suse.de