drm/pl111: Use drm_fb_cma_fbdev_init/fini()

Use drm_fb_cma_fbdev_init() and drm_fb_cma_fbdev_fini() which relies on
the fact that drm_device holds a pointer to the drm_fb_helper structure.
This means that the driver doesn't have to keep track of that.
Also use the drm_fb_helper functions directly.

Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20171115142001.45358-11-noralf@tronnes.org

drivers/gpu/drm/pl111/pl111_drm.h

@@ -53,7 +53,6 @@ struct pl111_drm_dev_private {
 	struct drm_panel *panel;
 	struct drm_bridge *bridge;
 	struct drm_simple_display_pipe pipe;
-	struct drm_fbdev_cma *fbdev;
 
 	void *regs;
 	u32 ienb;