drm/amdgpu: Fix per-IB secure flag GFX hang

Since commit "Move to a per-IB secure flag (TMZ)",
we've been seeing hangs in GFX. We need to send
FRAME CONTROL stop/start back-to-back, every time
we flip the TMZ flag. That is, when we transition
from TMZ to non-TMZ we have to send a stop with
TMZ followed by a start with non-TMZ, and
similarly for transitioning from non-TMZ into TMZ.

This patch implements this, thus fixing the GFX
hang.

v1 -> v2:
As suggested by Luben, and accept part of implemetation from this patch:
- Put "secure" closed to the loop and use optimization
- Change "secure" to bool again, and move "secure == -1" out of loop.
v3: Small fixes/optimizations.

Reported-and-Tested-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>