drm: Remove unnecessary drm_panel_attach and drm_panel_detach

drm: Remove unnecessary drm_panel_attach and drm_panel_detach

These functions are now empty and no longer
useful so remove the functions and their uses.

Signed-off-by: Joe Perches <joe@perches.com>
Cc: Bernard Zhao <bernard@vivo.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>,
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Icenowy Zheng <icenowy@aosc.io>,
Cc: Jagan Teki <jagan@amarulasolutions.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Robert Chiras <robert.chiras@nxp.com>
Cc: dri-devel@lists.freedesktop.org,
Cc: linux-kernel@vger.kernel.org
Cc: opensource.kernel@vivo.com
Signed-off-by: Sam Ravnborg <sam@ravnborg.org> # Fixed build and a few warnings
Link: https://patchwork.freedesktop.org/patch/msgid/9e13761020750b1ce2f1fabee23ef6e2a2942882.camel@perches.com