msm: camera: Do not increment refcount when buffer is already mapped

When getting the IOVA from camera SMMU driver, if the buffer IOVA is
already mapped, then do not increment the refcount. If refcount is
incremented, when unmapping the buffer, it won't be actually unmapped
because the non zero refcount will prevent it from getting unmapped
from the context bank.

CRs-Fixed: 1108641
Change-Id: I46e114f90d47cc003231e234f6f06d9e1fdca652
Signed-off-by: Seemanta Dutta <seemanta@codeaurora.org>