msm: ipa: fix for DMA memory allocation failures

Use the 2 descriptor ipa_tx DMA pool for 2 descriptor transfers
and coherent allocation for everything else. This will reduce
odds of DMA allocation failures on SW data path due to small
atomic DMA coherent pool size.

Change-Id: I4530cc3a76f7a9b16a3d0fe004f214a46e763822
Signed-off-by: Ravi Gummadidala <rgummadi@codeaurora.org>
Signed-off-by: Talel Shenhar <tatias@codeaurora.org>