mm-video: vidc: Restore sanity representing buffer sizes in dyn buf mode

nFilledLen and nAllocLen are buffer handle sizes in dynamic buffer mode.
This led to obnoxious checks in multiple places to account for dynamic
buffer mode.  Furthermore it led to basic sanity checks such as
`nFilledLen + nOffset < nAllocLen` failing.

To get around this, overwrite the OMX_BUFFERHEADER's nFilledLen /
nAllocLen parameters on FTB and restore them on FBD.  This localises the
need for the aforementioned irritating check.

CRs-Fixed: 647383
Change-Id: I38d5a1ef88d90f3f12164c48b60ac3bc45d50b0a