backend-drm: do not print head info twice

Before this patch, when a new head is found its information is printed
first as "updated" and then as "found" in the log.

The reason is that drm_head_create() calls drm_head_update_info() which
printed the head as "changed". Then drm_head_create() itself prints it
as "found".

This fixes it to print only once as "found".

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>