drm/i915: Document BDW+ DRRS M/N programming requirements

When reprogramming M/N live on BDW+ we must write the LINK_N
register last as it's the one that arms the double buffered
register update for all the M/N registers. Document this so
that we don't accidentally break things.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220128103757.22461-18-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/display/intel_display.c

@@ -3152,6 +3152,10 @@ void intel_set_m_n(struct drm_i915_private *i915,
 	intel_de_write(i915, data_m_reg, TU_SIZE(m_n->tu) | m_n->data_m);
 	intel_de_write(i915, data_n_reg, m_n->data_n);
 	intel_de_write(i915, link_m_reg, m_n->link_m);
+	/*
+	 * On BDW+ writing LINK_N arms the double buffered update
+	 * of all the M/N registers, so it must be written last.
+	 */
 	intel_de_write(i915, link_n_reg, m_n->link_n);
 }