drm/amdgpu: fix locking in vega10_ih_prescreen_iv

The vm pointer can become invalid as soon as the lock is released.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>