drm/i915: Also discard second CRC on gen8+ platforms.

One of the differences I spotted for GEN8+ platforms when
compared to older platforms is that spec for BDW+ includes
this sentence:

"The first CRC done indication after CRC is first enabled is
from only a partial frame, so it will not have the expected
CRC result."

This is an indication that on BDW+ platforms, by the time
we receive the interrupt the CRC is not accurate yet for
the full frame. That would be ok, because we are already
skipping the first CRC for all platforms. However the comment
on the code state that it is for some unknown reason. Also,
on CHV (gen8 lp) we were already discarding the second CRC
as well to make sure we have a reliable CRC on hand.

So based on all ou tests and bugs it seems that it is not
on CHV that needs to discard 2 first CRCs, but all BDW+
platforms.

Starting on SKL we have this CRC done bit (24), but the
experiments around the use of this bit wasn't that stable
as just discarding the second CRC. So, let's for now
just move with CHV solution for all gen8+ platforms and
make our CI a bit more stable.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102374
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101309
Cc: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Tested-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170928002040.7917-1-rodrigo.vivi@intel.com

drivers/gpu/drm/i915/i915_irq.c

@@ -1647,11 +1647,11 @@ static void display_pipe_crc_irq_handler(struct drm_i915_private *dev_priv,
 		 * bonkers. So let's just wait for the next vblank and read
 		 * out the buggy result.
 		 *
-		 * On CHV sometimes the second CRC is bonkers as well, so
+		 * On GEN8+ sometimes the second CRC is bonkers as well, so
 		 * don't trust that one either.
 		 */
 		if (pipe_crc->skipped == 0 ||
-		    (IS_CHERRYVIEW(dev_priv) && pipe_crc->skipped == 1)) {
+		    (INTEL_GEN(dev_priv) >= 8 && pipe_crc->skipped == 1)) {
 			pipe_crc->skipped++;
 			spin_unlock(&pipe_crc->lock);
 			return;