Use commit_dcache() instead of commit_discard_idcache() where possible

- PCM playback DMA doesn't need the discard. Only recording DMA would
  need it.
- When creating threads for another core, the core creating the thread only
  needs to commit. The discard on the other core is handled elsewhere.

Change-Id: I864a0777e22f221a66218efd2c02ff3ad3889736

firmware/thread.c

@@ -1609,7 +1609,7 @@ unsigned int create_thread(void (*function)(void),
     /* Writeback stack munging or anything else before starting */
     if (core != CURRENT_CORE)
     {
-        commit_discard_idcache();
+        commit_dcache();
     }
 #endif