drm/i915/display: Add fixed_rr to crtc_state dump

Add fixed refresh rate mode in crtc_state dump.
VRR Timing Generator is running in fixed refresh rate mode when
vrr.vmin = vrr.vmax = vrr.flipline.

v2: s/fixed_rr/fixed rr for consistency with the other stuff. (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/20250322044345.3827137-2-ankit.k.nautiyal@intel.com