drm/i915/vrr: Fix intel_vrr_always_use_vrr_tg()==true on TGL

drm/i915/vrr: Fix intel_vrr_always_use_vrr_tg()==true on TGL

On TGL the hardware always needs TRANS_VBLANK.VBLANK_START
to be programemd with VACTIVE+SCL. Make it so.

The current way of programming it with crtc_vblank_start only
works for the legacy timing generator, as there the delayed
vblank does happen exactly at VACTIVE+SCL.

But if one tries to change intel_vrr_always_use_vrr_tg() to
always use the VRR timing generator on TGL, crtc_vblank_start
will point to the VRR timing generator's delayed vblank,
which may not match VACTIVE+SCL.

Fortunately the state checker caught the issue right away
when I tried intel_vrr_always_use_vrr_tg()==true on TGL.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20251020185038.4272-2-ville.syrjala@linux.intel.com
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>