drm/ast: Map HW cursor BOs permanently

The BOs of the hardware cursor are now mapped permanently while the
cursor plane is being used. This reduces the CPU overhead of the cursor
plane's atomic_update function.

The change also resolves a problem with the vmap call in the commit tail.
The vmap implementation could acquire the DMA reservation lock on the
BO, which is not allowed that late in the atomic update. Removing the
vmap call from atomic_update fixes the issue.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209134632.12157-9-tzimmermann@suse.de