drm/amd/display: clear request when release aux engine

[Why]
when driver and dmub request aux engine at the same time,
dmub grant the aux engine but driver fail. Then driver
release aux engine but doesn't clear the request bit.
Then aux engine will be occupied by driver forever.

[How]
When driver release aux engine, clear request bit as well.

Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Paul Hsieh <paul.hsieh@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>