drm/amdgpu/gfx10: explicitly wait for cp idle after halt/unhalt

50us is not enough to wait for cp ready after gpu reset on some navi asics.

Signed-off-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Suggested-by: Jack Xiao <Jack.Xiao@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>