drm/crtc: Introduce drmm_crtc_init_with_planes

The DRM-managed function to register a CRTC is
drmm_crtc_alloc_with_planes(), which will allocate the underlying
structure and initialisation the CRTC.

However, we might want to separate the structure creation and the CRTC
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 a CRTC that would be passed as
an argument.

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