drm/imx: cleanup the imx-drm header

Drop unused defines and obsolete prototypes from the imx-drm.h header.

Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Tested-by: Chris Healy <cphealy@gmail.com>
Tested-by: Philipp Zabel <p.zabel@pengutronix.de> # on imx6q-nitrogen6x
Link: https://patchwork.freedesktop.org/patch/msgid/20240602-drm-imx-cleanup-v3-3-e549e2a43100@linaro.org


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>