msm: vidc: Simplify buffer map and unmap sequence

Video driver will map the buffer twice in ETB/FTB. First map will
call iommu_map and following maps will just increment the refcount.
First unmap will happen in EBD/FBD and second unmap will happen if
READONLY flag is not present in EBD/FBD. If READONLY flag is not
present in EBD/FBD then video hardware will send RBR event
(Release Buffer Reference) to driver where second unmap will happen.
Though video driver calls iommu_unmap for second unmap the buffer
is not actually unmapped in iommu driver as video buffers are mapped
using late unmap feature. The buffer will get unmapped in iommu driver
when it is freed.

Change-Id: Ic0043ef97146e3b1081f2fc0fc3da715396be1a0
Signed-off-by: Maheshwar Ajja <majja@codeaurora.org>