Forward declare struct drm_printer and include <linux/err.h>.

v2: Include <linux/err.h> (kernel test robot)

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240307083410.2604712-1-jani.nikula@intel.com