drm/amdgpu: use the callback function for reset status polling on IMU

Switch to use the callback function to poll the reset status on IMU.
Because it will have different sequency on other ASICs.

v2: drop unused variable (Alex)

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>