drm/vkms: Switch to managed for encoder

The current VKMS driver uses non-managed function to create encoders. It
is not an issue yet, but in order to support multiple devices easily,
convert this code to use drm and device managed helpers.

Reviewed-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: José Expósito <jose.exposito89@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250116-google-vkms-managed-v9-2-3e4ae1bd05a0@bootlin.com


Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>