drm/i915: Introduce intel_mode_vdisplay()

The DSB code will need to know the hardware's idea of vertical
active, as that is also what defines the start of undelayed
vblank. Introduce a helper that gives us that information,
in line with the other intel_mode_v*() functions.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240611133344.30673-5-ville.syrjala@linux.intel.com


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