drm/amd/display: Add hardware reset interface for DMUB service

[Why]
We'll need this to perform a clean shutdown before unloading the driver.

[How]
It will call reset internally and set hw_init to false. It won't do
anything if the hardware isn't initialized.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>