msm: kgsl: Elevate the priority of the thread during adreno start

It turns out that having lots of spurious threads causes race
conditions. Who knew? Instead of spawning a new high prority
thread just bump the priority of the current thread up
while adreno_start is running.

Change-Id: Ic0dedbadf65da11c800578c8769ae844f010e925
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>