drm/msm/a6xx: Remove an unused struct member

The HFI tasklet was removed in df0dff13 ("drm/msm/a6xx: Poll for HFI
responses") but the tasklet_struct was accidentally left behind.

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