drm/panel: remove unnecessary forward declaration

'struct drm_device' is not used at all in this file since commit
aa6c4364 ("drm/panel: drop drm_device from drm_panel").

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250206-drm-cleanups-v1-1-93df60faa756@bootlin.com