Like how we handle overlays, we split drm_output_set_cursor() into
drm_output_prepare_cursor_surface() that will assign the hw cursor to
a suitable surface and drm_output_set_cursor() that will do the actual
work of updating cursor contents and moving it.  This is more in line
with how we handle everything else, and lets us update cursor contents
based on changes to the cursor plane damage.

This works when the cursor content changes but also accumulates
damage when the cursor changes output.  In that case we move the
cursor surface to the cursor plane in the new output and
weston_surface_move_to_plane() will generate the damage that triggers
download of sprite contents there.