msm: ipa: replace dma_zalloc_coherent()

dma_zalloc_coherent() is no longer needed and get deprecated as it has
no users because dma_alloc_coherent() already zeroes out memory for us.

Change-Id: Ibd49d7afce8caaa7b2b6aed61884b307674f9a59
Signed-off-by: Bojun Pan <bojunp@codeaurora.org>