drm/encoder: Introduce drmm_encoder_init

The DRM-managed function to register an encoder is
drmm_encoder_alloc() and its variants, which will allocate the underlying
structure and initialisation the encoder.

However, we might want to separate the structure creation and the encoder
initialisation, for example if the structure is shared across multiple DRM
entities, for example an encoder and a connector.

Let's create an helper to only initialise an encoder that would be passed
as an argument.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>