Having a cursor move function is misleading, as it actually enables the
cursor's image for displaying. So rename it to ast_cursor_show(). It's
semantics is to show a cursor at the specified location on the screen.
The displayed cursor is always the image in the cursor front BO.

This change also simplifies struct ast_crtc to being a mere wrapper around
around struct drm_crtc. It will be removed by a later patch.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200702115029.5281-7-tzimmermann@suse.de