drm/amd/pm: re-enable the gfx imu when smu resume

If the gfx imu is poweroff when suspend, then
it need to be re-enabled when resume.

Signed-off-by: Tim Huang <tim.huang@amd.com>
Reviewed-by: Yifan Zhang <yifan1.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>