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

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

Fixes: 47368ab ("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>

Author: lumag

include/drm/display/drm_hdmi_state_helper.h

@@ -6,6 +6,7 @@
 struct drm_atomic_state;
 struct drm_connector;
 struct drm_connector_state;
+struct drm_display_mode;
 struct hdmi_audio_infoframe;
 
 enum drm_connector_status;