msm: kgsl: Avoid another race condition with GPU halt

A race condition may occur while clearing GPU halt
from _adreno_start() in one thread and at the same
time decrementing it inside adreno_put_gpu_halt()
from adreno_dispatcher_idle_unsafe() in the other
thread. This can cause BUG_ON as "halt" value would
become negative.

Clearing GPU halt inside _adreno_start() should not
be required explicitly for below reasons:
1. "halt" is member of static global variable, so it
would be initialized to zero at the beginning.
2. Commands submission to GPU from the thread executing
_adreno_start() should be blocked until "halt" is reset
back to zero from adreno_dispatcher_idle_unsafe().

CRs-Fixed: 678633
Change-Id: I06bbf4ad751a72b2e943365c1ac1952f67ac299c
Signed-off-by: Prabhat Awasthi <pawasthi@codeaurora.org>