Fallback to dma_alloc_coherent if shmbridge is disabled

If shmbridge is not enabled, allocate memory through
dma_alloc_coherent instead of shmbridge pool. This will make the
clients independent of whether shmbridge is enabled or
disabled.

Change-Id: I293872a2e57dec77d660de1bd857a425d4888a5b
Signed-off-by: Anmolpreet Kaur <anmolpre@codeaurora.org>