drm/amd/display: Wait for clean shutdown in DMCUB reset

[Why]
The DMCUB may be currently executing commands when the reset is
triggered.

Before issuing a reset we should first wait for the DMCUB to finish
its work.

[How]
Send the GPINT command for halting the firmware before reset.

Get the ack for the command then wait for the scratch register to
become the correct value.

We want this to take under ~40us or so at most before we force reset
to cover PHY delay sequence max time.

Each register read will be at least ~1-3us so don't bother using udelay.

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>