drm/vkms: Remove <drm/drm_simple_kms_helper.h> include

The driver doesn't use simple-KMS helpers to set a simple display pipeline
but only the drm_simple_encoder_init() function to initialize an encoder.

That helper is just a wrapper of drm_encoder_init(), but passing a struct
drm_encoder_funcs that sets the .destroy handler to drm_encoder_cleanup().

Since the <drm/drm_simple_kms_helper.h> header is only included for this
helper and because the connector is initialized with drm_connector_init()
as well, do the same for the encoder and drop the header include.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230406110235.3092055-3-javierm@redhat.com