drm/amdgpu: rework lock handling for flush_tlb v2

Instead of each implementation doing this more or less correctly
move taking the reset lock at a higher level.

v2: fix typo

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