mm-video: vidc: Make buffer state bitmap 64 bits

Previously the buffer state bitmap (which indicates if the given buffer
at an index is free or allocated) was unsigned int, which was typically
32 bits.  However if the client wants more than 32 buffers, the state
can't be held within the 32-bit bitmap.  Hence change it to uint64_t.

Change-Id: I8403c1331352bb96570577362a46444600d88448
mm-video: vidc: Avoid setting invalid timestamp of eos NAL

In arbitrary mode when eos NAL is appened at the end, while
coalscing with destination buffer, the actual timestamp is
overwritten with LLONG_MAX. This will lead to error log and
setting the flag V4L2_QCOM_BUF_TIMESTAMP_INVALID in driver,
which is not needed.

Change-Id: I87a63c4cb9d8259280b781cdd553050580809e8e