drm/imx: add forward declarations for types

The imx.h header does not forward declare the types it uses, and the
header is not self-contained. Fix it.

Fixes: cc3e8a216d6b ("drm/imx: add internal bridge handling display-timings DT node")
Cc: Philipp Zabel <[e-mail]>
Cc: Dmitry Baryshkov <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Reviewed-by: Philipp Zabel <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
Signed-off-by: Jani Nikula <[e-mail]>

include/drm/bridge/imx.h

···
 #ifndef DRM_IMX_BRIDGE_H
 #define DRM_IMX_BRIDGE_H
 
+struct device;
+struct device_node;
+struct drm_bridge;
+
 struct drm_bridge *devm_imx_drm_legacy_bridge(struct device *dev,
 					      struct device_node *np,
 					      int type);