msm: vidc: Allocate ocmem immediately after initialising f/w

We're freeing ocmem just prior to unloading the firmware but allocating
it after intialising a session.  This is somewhat asymmetric and makes
the code confusing to follow.

Hence allocate it just after initialising the firmware.  Since ocmem is
a core resource, we don't need an active session to allocate and provide
to firmware.

Change-Id: I3673b5506e165ac4558b3d820f31e4bba7f24047
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>