drm/amdgpu: IH process reset count when restart

Otherwise when IH process restart, count is zero, the loop will
not exit to wake_up_all after processing AMDGPU_IH_MAX_NUM_IVS
interrupts.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>