drm/i915: Check idle to active before processing CSQ

If idle to active bit is set, the rest of the fields
in CSQ are not valid.

Bail out early if this is the case in order to prevent
rest of the loop inspecting stale values.

This was found by Bspec/code inspection. Doesn't seem to fix any of
the known issues.
Signed-off-by: NMika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: NArun Siluvery <arun.siluvery@linux.intel.com>
[danvet: Add note about how this was found.]
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_lrc.c

@@ -497,6 +497,9 @@ void intel_lrc_irq_handler(struct intel_engine_cs *ring)
 		status_id = I915_READ(RING_CONTEXT_STATUS_BUF(ring) +
 				(read_pointer % 6) * 8 + 4);
 
+		if (status & GEN8_CTX_STATUS_IDLE_ACTIVE)
+			continue;
+
 		if (status & GEN8_CTX_STATUS_PREEMPTED) {
 			if (status & GEN8_CTX_STATUS_LITE_RESTORE) {
 				if (execlists_check_remove_request(ring, status_id))