drm/i915/vblank: Add helper to get correct vblank length

drm/i915/vblank: Add helper to get correct vblank length

Currently crtc_vblank_start is assumed to be the vblank_start for the fixed
refresh rate case. That value can be different from the variable refresh
rate case whenever always_use_vrr_tg()==false. On icl/tgl it's always
different due to the extra vblank delay, and also on adl+ it could be
different if we were to use an optimized guardband.

So places where crtc_vblank_start is used to compute vblank length needs
change so as to account for cases where vrr is enabled. Specifically
with vrr.enable the effective vblank length is actually guardband.

Add a helper to get the correct vblank length for both vrr and fixed
refresh rate cases. Use this helper where vblank_start is used to
compute the vblank length.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/20251016055415.2101347-5-ankit.k.nautiyal@intel.com