msm: vidc: Fix decoder input buffer size calculation

Decoder input buffer size is currently calculated in MBs
instead of actual size. Multiply MB size with num pixels in
MB.

CRs-Fixed: 2368458
Change-Id: Ia0a2add18780d026d3372526e19575c89c5169e1
Signed-off-by: Vaibhav Deshu Venkatesh <vdeshuve@codeaurora.org>