drm/vc4: crtc: Move the CRTC initialisation to a separate function

The upcoming patches to turn the TXP into a full-blown CRTC will have the
same CRTC initialisation code, so let's move it into a separate, public,
function so that we can reuse it later on.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/3a3026c0e7408895d154d8dea454cf6d1c459715.1591882579.git-series.maxime@cerno.tech

drivers/gpu/drm/vc4/vc4_drv.h

@@ -794,6 +794,9 @@ void vc4_bo_remove_from_purgeable_pool(struct vc4_bo *bo);
 
 /* vc4_crtc.c */
 extern struct platform_driver vc4_crtc_driver;
+int vc4_crtc_init(struct drm_device *drm, struct vc4_crtc *vc4_crtc,
+		  const struct drm_crtc_funcs *crtc_funcs,
+		  const struct drm_crtc_helper_funcs *crtc_helper_funcs);
 void vc4_crtc_destroy(struct drm_crtc *crtc);
 int vc4_page_flip(struct drm_crtc *crtc,
 		  struct drm_framebuffer *fb,