drm/i915/display: Squelch kerneldoc warnings

Just a parameter rename,

drivers/gpu/drm/i915/display/intel_display.c:14425: warning: Function parameter or member '_new_plane_state' not described in 'intel_prepare_plane_fb'
drivers/gpu/drm/i915/display/intel_display.c:14425: warning: Excess function parameter 'new_state' description in 'intel_prepare_plane_fb'
drivers/gpu/drm/i915/display/intel_display.c:14534: warning: Function parameter or member '_old_plane_state' not described in 'intel_cleanup_plane_fb'
drivers/gpu/drm/i915/display/intel_display.c:14534: warning: Excess function parameter 'old_state' description in 'intel_cleanup_plane_fb'

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191012080208.18774-1-chris@chris-wilson.co.uk