drm/i915: Add comments about fixed pipe->transcoder/PLL mapping

Code checkers may complain about the explicit casts between different
enum types, so add comments for known-valid cases to help future
triaging of such complaints.

v2:
- Make the comments more logical (Ville).
Signed-off-by: NImre Deak <imre.deak@intel.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463059132-1720-3-git-send-email-imre.deak@intel.com

drivers/gpu/drm/i915/intel_display.c

@@ -9290,6 +9290,10 @@ static bool ironlake_get_pipe_config(struct intel_crtc *crtc,
 		ironlake_get_fdi_m_n_config(crtc, pipe_config);
 
 		if (HAS_PCH_IBX(dev_priv)) {
+			/*
+			 * The pipe->pch transcoder and pch transcoder->pll
+			 * mapping is fixed.
+			 */
 			pll_id = (enum intel_dpll_id) crtc->pipe;
 		} else {
 			tmp = I915_READ(PCH_DPLL_SEL);
@@ -9840,6 +9844,10 @@ static bool hsw_get_transcoder_state(struct intel_crtc *crtc,
 	enum intel_display_power_domain power_domain;
 	u32 tmp;
 
+	/*
+	 * The pipe->transcoder mapping is fixed with the exception of the eDP
+	 * transcoder handled below.
+	 */
 	pipe_config->cpu_transcoder = (enum transcoder) crtc->pipe;
 
 	/*