msm: vidc: Clean up session handling for 64 bit driver support

Instead of passing the session pointer value to fw, hash it to a
32 bit value and pass that.
Clean up the function to validate session pointer from the packet
received from fw.

Change-Id: Iff82ba9b81ddf544add570b62a7c0d6b0f5e32a2
Signed-off-by: Shalaj Jain <shalajj@codeaurora.org>