drm/amd/display: Release dm_state->context when state is cleared.

Handling a use case of TEST_ONLY request from DRM where commit is
not goiing to be called. We need to release the allocated dc_validate_context
in this case.

Signed-off-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>