drm/i915/vrr: Track vrr.enable only for variable timing

Since CMRR is now disabled, use the flag vrr.enable to tracks if vrr timing
generator is used with variable timings.

Avoid setting vrr.enable for CMRR and adjust readout to not set vrr.enable
when vmax == vmin == flipline (fixed refresh rate timing).

v2: Use intel_vrr_vmin_flipline() to account for adjustments required
for icl/tgl. (Ville)

v3: Add a #TODO for handling I915_MODE_FLAG_VRR better for CMRR. (Ville)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250311093751.1329043-6-ankit.k.nautiyal@intel.com