drm/i915/vrr: Prepare for fixed refresh rate timings

drm/i915/vrr: Prepare for fixed refresh rate timings

Currently we always compute the timings as if vrr is enabled.
With this approach the state checker becomes complicated when we
introduce fixed refresh rate mode with vrr timing generator.

To avoid the complications, instead of always computing vrr timings, we
compute vrr timings based on uapi.vrr_enable knob.
So when the knob is disabled we always compute vmin=flipline=vmax.

v2: Use actual timings without any adjustments while preparing for
fixed timings in compute_config. (Ville)
v3: Avoid setting fixed timings if !vrr_possible().
v4: Move vmin adjustement after all other timings are complete. (Ville)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (#v2)
Link: https://patchwork.freedesktop.org/patch/msgid/20250311093751.1329043-8-ankit.k.nautiyal@intel.com