drm/panel: make drm_panel.h self-contained

Fix build warning if drm_panel.h is built with CONFIG_OF=n or
CONFIG_DRM_PANEL=n and included without the prerequisite err.h:

./include/drm/drm_panel.h: In function ‘of_drm_find_panel’:
./include/drm/drm_panel.h:203:9: error: implicit declaration of function ‘ERR_PTR’ [-Werror=implicit-function-declaration]
  return ERR_PTR(-ENODEV);
         ^~~~~~~
./include/drm/drm_panel.h:203:9: error: returning ‘int’ from a function with return type ‘struct drm_panel *’ makes pointer from integer without a cast [-Werror=int-conversion]
  return ERR_PTR(-ENODEV);
         ^~~~~~~~~~~~~~~~

Fixes: 5fa8e4a22182 ("drm/panel: Make of_drm_find_panel() return an ERR_PTR() instead of NULL")
Signed-off-by: Jani Nikula <[email protected]>
Acked-by: Thierry Reding <[email protected]>
Reviewed-by: Sam Ravnborg <[email protected]>
Reviewed-by: Sean Paul <[email protected]>
Cc: Boris Brezillon <[email protected]>
Signed-off-by: Sam Ravnborg <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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