drm/ast: Only map cursor BOs during updates

The HW cursor's BO used to be mapped permanently into the kernel's
address space. GEM's vmap operation will be protected by locks, and
we don't want to lock the BO's for an indefinate period of time.

Change the cursor code to map the HW BOs only during updates. The
vmap operation in VRAM helpers is cheap, as a once estabished mapping
is being reused until the BO actually moves. As the HW cursor BOs are
permanently pinned, they never move at all.

v2:
	* fix typos in commit description

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201209142527.26415-3-tzimmermann@suse.de