msm: VPU: Implement Session Create and Session Join Ioctls

New ioctls ensure no overlap in VPU usage from independent clients.
- VPU_CREATE_SESSION ioctl allows clients to create a new session by
attaching to the next available one.
- VPU_JOIN_SESSION allows clients to join an active session.
- VPU_ATTACH_TO_SESSION is deprecated but remains unchanged.
New events related to session attach/detach to allow clients to coordinate
use of VPU.

Change-Id: Icb22461824daa97c06bf54fa66b67156fbfa0b93
Signed-off-by: Ahmed Abdul-Salam <aabdulsa@codeaurora.org>