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We have several copies of code calculating the hardware's
idea of vtotal. Pull that to a helper, similar to
intel_mode_vblank_{start,end}().
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240528185647.7765-4-ville.syrjala@linux.intel.com

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