drm/ast: Allocate cursor BOs at high end of video memory

By putting cursor BOs at the high end of the video memory, we can avoid
memory fragmentation. Starting at the low end, contiguous video memory is
available for framebuffers.

The patch also simplifies the buffer swapping by splitting
struct ast_private.cursor_cache BO into two separate boffer objects. Cursor
images alternate between these buffers instead of offsets within cursor_cache.

v3:
	* fixes space-before-tab error near AST_HWC_SIGNATURE_CHECKSUM

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190927090309.10254-6-tzimmermann@suse.de