drm/mgag200: Rewrite cursor handling

The cursor handling in mgag200 is complicated to understand. It touches a
number of different BOs, but doesn't really use all of them.

Rewriting the cursor update reduces the amount of cursor state. There are
two BOs for double-buffered HW updates. The source BO updates the one that
is currently not displayed and then switches buffers. Explicit BO locking
has been removed from the code. BOs are simply pinned and unpinned in video
RAM.

v2:
	* pin cursor BOs to current location

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190613073041.29350-8-tzimmermann@suse.de