drm/amdgpu: support imu for gfx11

Add support to initialize imu for gfx v11.
IMU is a new power management block for
gfx which manages gfx power.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>