drm/i915: Actually retry with bit-banging after GMBUS timeout

After the GMBUS transfer times out, we set force_bit=1 and
return -EAGAIN expecting the i2c core to call the .master_xfer
hook again so that we will retry the same transfer via bit-banging.
This is in case the gmbus hardware is somehow faulty.

Unfortunately we left adapter->retries to 0, meaning the i2c core
didn't actually do the retry. Let's tell the core we want one retry
when we return -EAGAIN.

Note that i2c-algo-bit also uses this retry count for some internal
retries, so we'll end up increasing those a bit as well.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: drm-intel-fixes@lists.freedesktop.org
Fixes: bffce907d6 ("drm/i915: abstract i2c bit banging fallback in gmbus xfer")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1457366220-29409-2-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
(cherry picked from commit 8b1f165a4a)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_i2c.c

@@ -664,6 +664,12 @@ int intel_setup_gmbus(struct drm_device *dev)
 
 		bus->adapter.algo = &gmbus_algorithm;
 
+		/*
+		 * We wish to retry with bit banging
+		 * after a timed out GMBUS attempt.
+		 */
+		bus->adapter.retries = 1;
+
 		/* By default use a conservative clock rate */
 		bus->reg0 = pin | GMBUS_RATE_100KHZ;