drm/i915: Enable VRR later during fastsets

In order to reconcile seamless M/N updates with VRR we'll
need to defer the fastset VRR enable to happen after the
seamless M/N update (which happens during the vblank evade
critical section). So just push the VRR enable to be the last
thing during the update.

This will also affect the vblank evasion as the transcoder
will now still be running with the old VRR state during
the vblank evasion. So just grab the timings always from the
old crtc state during any non-modeset commit, and also grab
the current state of VRR from the active timings (as we disable
VRR before vblank evasion during fastsets).

This also fixes vblank evasion for seamless M/N updates as
we now properly account for the fact that the M/N update
happens after vblank evasion.

Cc: Manasi Navare <navaremanasi@chromium.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230901130440.2085-5-ville.syrjala@linux.intel.com
Reviewed-by: Manasi Navare <navaremanasi@chromium.org>
Reviewed-by: Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>