drm/i915: clarify why we need to enable fdi plls so early

For reference, see "Graphics BSpec: vol4g North Display Engine
Registers [IVB], Display Mode Set Sequence", step 4 of the enabling
sequence:

a. "Enable PCH FDI Receiver PLL, wait for warmup plus DMI latency
b. "Switch from Rawclk to PCDclk in FDI Receiver
c. "Enable CPU FDI Transmitter PLL, wait for warmup"

Cc: Paulo Zanoni <przanoni@gmail.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -3227,6 +3227,9 @@ static void ironlake_crtc_enable(struct drm_crtc *crtc)
 	is_pch_port = ironlake_crtc_driving_pch(crtc);
 
 	if (is_pch_port) {
+		/* Note: FDI PLL enabling _must_ be done before we enable the
+		 * cpu pipes, hence this is separate from all the other fdi/pch
+		 * enabling. */
 		ironlake_fdi_pll_enable(intel_crtc);
 	} else {
 		assert_fdi_tx_disabled(dev_priv, pipe);