msm: vidc: Optimize IOMMU map size for H264 decoder

IOMMU map size is not required to be double the actual
buffer size for H264 decoder. So reducing IOMMU map
size for H264 format which will enable support for
two H264 decoder instances in parallel in smooth streamig
mode where the number of output buffers equal to 18 with
each buffer size equal to 1080p buffer size.

CRs-fixed: 491718
Change-Id: Ie6782b2ff58667000213f25482eea151b849762b
Signed-off-by: Maheshwar Ajja <majja@codeaurora.org>