drm/amdgpu: let mode2 reset fallback to default when failure

- introduce AMDGPU_SKIP_MODE2_RESET flag
- let mode2 reset fallback to default reset method if failed

v2: move this part out from the asic specific part

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>