sched: walt: Refactor WALT initialization routine

The global variables are initialized in walt_sched_init(). This
function is called for each rq initialization and the global
variables are initialized again unnecessarily. Refactor the code
to fix this.

Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
Change-Id: I085bc9ff576f655afa12fda015f054fbab3a0c91