drm/crtc: make drm_crtc_internal.h self-contained

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

drivers/gpu/drm/drm_crtc_internal.h

@@ -32,6 +32,7 @@
  * and are not exported to drivers.
  */
 
+#include <linux/err.h>
 #include <linux/types.h>
 
 enum drm_color_encoding;
@@ -54,6 +55,7 @@ struct drm_mode_object;
 struct drm_mode_set;
 struct drm_plane;
 struct drm_plane_state;
+struct drm_printer;
 struct drm_property;
 struct edid;
 struct fwnode_handle;