drm/i915: fix compiler warning in drivers/gpu/drm/i915/intel_uncore.c

When building with gcc-7, the following warning happens:

drivers/gpu/drm/i915/intel_uncore.c: In function ‘hsw_unclaimed_reg_detect’:
drivers/gpu/drm/i915/intel_uncore.c:638:36: warning: decrement of a boolean expression [-Wbool-operation]
   i915.mmio_debug = mmio_debug_once--;
                                    ^~

As it's really not wise to -- on a boolean value.

Commit 75714940 ("drm/i915: Do one shot unclaimed mmio detection
less frequently") which showed up in 4.6-rc1 does solve this issue, by
rewriting the mmio detection logic, but that isn't really good to
backport to 4.4-stable, so just fix up the obvious logic here to do the
right thing.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Paulo Zanoni <przanoni@gmail.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/i915/intel_uncore.c

@@ -635,7 +635,8 @@ hsw_unclaimed_reg_detect(struct drm_i915_private *dev_priv)
 			  "enabling oneshot unclaimed register reporting. "
 			  "Please use i915.mmio_debug=N for more information.\n");
 		__raw_i915_write32(dev_priv, FPGA_DBG, FPGA_DBG_RM_NOCLAIM);
-		i915.mmio_debug = mmio_debug_once--;
+		i915.mmio_debug = mmio_debug_once;
+		mmio_debug_once = false;
 	}
 }