drm/i915: Add code comment on assumption of pipe==transcoder

Add a comment to the pipe and transcoder enum definitions about our
assumption in the code about enum values for pipes and transcoders
with a 1:1 transcoder -> pipe mapping.

v2:
- Clarify more what are the assumptions about the enum values. (Ville)
v3: (Lucas)
- s/->/ -> / so it looks less like pointer dereferencing.
- Use pipe enums as initializers in the transcoder enum definition.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (v2)
Link: https://patchwork.freedesktop.org/patch/msgid/20181120092325.21249-3-imre.deak@intel.com

drivers/gpu/drm/i915/intel_display.h

@@ -43,6 +43,11 @@ enum i915_gpio {
 	GPIOM,
 };
 
+/*
+ * Keep the pipe enum values fixed: the code assumes that PIPE_A=0, the
+ * rest have consecutive values and match the enum values of transcoders
+ * with a 1:1 transcoder -> pipe mapping.
+ */
 enum pipe {
 	INVALID_PIPE = -1,
 
@@ -57,9 +62,20 @@ enum pipe {
 #define pipe_name(p) ((p) + 'A')
 
 enum transcoder {
-	TRANSCODER_A = 0,
-	TRANSCODER_B,
-	TRANSCODER_C,
+	/*
+	 * The following transcoders have a 1:1 transcoder -> pipe mapping,
+	 * keep their values fixed: the code assumes that TRANSCODER_A=0, the
+	 * rest have consecutive values and match the enum values of the pipes
+	 * they map to.
+	 */
+	TRANSCODER_A = PIPE_A,
+	TRANSCODER_B = PIPE_B,
+	TRANSCODER_C = PIPE_C,
+
+	/*
+	 * The following transcoders can map to any pipe, their enum value
+	 * doesn't need to stay fixed.
+	 */
 	TRANSCODER_EDP,
 	TRANSCODER_DSI_0,
 	TRANSCODER_DSI_1,