drm/amdgpu: add debugfs amdgpu_reset_level

Introduce amdgpu_reset_level debugfs in order to help debug and
test specific type of reset. Also helps blocking unwanted type of
resets.

By default, mode2 reset will not be enabled

v2: make this debugfs in adev and use debugfs_create_u32

Signed-off-by: Victor Zhao <Victor.Zhao@amd.com>
Acked-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>