drm/i915: Extract intel_mode_vblank_delay()

Extract the code that computes the hardware centric view
of the vblank delay into a helper. We'll need a slightly
different variant for VRR soon.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241210211007.5976-10-ville.syrjala@linux.intel.com


Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>