drm/msm/a6xx: Poll for HFI responses

The only HFI communication with the GMU on sdm845 happens
during initialization and all commands are synchronous. A fancy
interrupt tasklet and associated infrastructure is entirely
not eeded and puts us at the mercy of the scheduler.

Instead poll for the message signal and handle the response
immediately and go on our way.

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