msm: kgsl: Change drawctxt mutex to spinlock

adreno_drawctxt_dump is called from within timer context and hence
this function cannot sleep. But it needs exclusive access to its
cmdqueue. Therefore, changing the drawctxt mutex to spinlock.

Change-Id: I180c3a4703ffc51bb9929df44dae862e2e6a432b
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>