drm/panel: drop drm_device from drm_panel

The panel drivers used drm_panel.drm for two purposes:
1) Argument to drm_mode_duplicate()
2) drm->dev was used in error messages

The first usage is replaced with drm_connector.dev
- drm_connector is already connected to a drm_device
  and we have a valid connector

The second usage is replaced with drm_panel.dev
- this makes drivers more consistent in their dev argument
  used for dev_err() and friends

With these replacements there are no more uses of drm_panel.drm,
so it is removed from struct drm_panel.
With this change drm_panel_attach() and drm_panel_detach()
no longer have any use as they are empty functions.

v2:
  - editorial correction in changelog (Laurent)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Cc: Stefan Mavrodiev <stefan@olimex.com>
Cc: Robert Chiras <robert.chiras@nxp.com>
Cc: "Guido Günther" <agx@sigxcpu.org>
Cc: Purism Kernel Team <kernel@puri.sm>
Link: https://patchwork.freedesktop.org/patch/msgid/20191207140353.23967-8-sam@ravnborg.org

include/drm/drm_panel.h

@@ -129,13 +129,6 @@ struct drm_panel_funcs {
  * struct drm_panel - DRM panel object
  */
 struct drm_panel {
-	/**
-	 * @drm:
-	 *
-	 * DRM device owning the panel.
-	 */
-	struct drm_device *drm;
-
 	/**
 	 * @dev:
 	 *