drm/msm/a6xx: Use new kernel API free function for gpu state

dadb36b7ec42 ("drm/msm: Add a common function to free kernel buffer objects")
missed freeing the crashdumper state for a6xx.

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