drm/i915: s/cstate/crtc_state/ in intel_get_frame_time_us()

Use standard variable name 'crtc_state' instead of 'cstate'.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231207193441.20206-8-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>