drm/display: hdmi-state-helper: add drm_display_mode declaration

Add forward-declaration for the struct drm_display_mode, missed in the
commit 47368ab4 ("drm/display: hdmi: add generic mode_valid helper")

Fixes: 47368ab4 ("drm/display: hdmi: add generic mode_valid helper")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250104-hdmi-state-display-mode-v1-1-3c06d36e726f@linaro.org


Signed-off-by: Maxime Ripard <mripard@kernel.org>