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: cc3e8a21 ("drm/imx: add internal bridge handling display-timings DT node")
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Cc: imx@lists.linux.dev
Cc: linux-arm-kernel@lists.infradead.org
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20241009140452.1981175-1-jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>