drm/ast: Keep cursor HW BOs mapped

Updating the image in a cursor's HW BO requires a mapping of the BO's
buffer in the kernel's address space. Cursor image updates can happen
frequently and create CPU overhead.

As cursor HW BOs are small and never move, they are now map exactly
once during the initialization and the mapping is used throughout the
driver's lifetime.

This change also removes a possible source of failures from
ast_cursor_show(). As the helper does not establish mappings, it cannot
fail. As a result, the cursor plane's atomic-update helper does not
call any failable interfaces. All failures are detected before trying
to update the cursor plane.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200702115029.5281-10-tzimmermann@suse.de