drm/msm/gpu: Rearrange the code that collects the task during a hang

Do a bit of cleanup to prepare for upcoming changes to pass the
hanging task comm and cmdline to the crash dump function.

v2: Use GFP_ATOMIC while holding the rcu lock per Chris Wilson

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>