drm/amdkfd: Support debugger in Navi1x trap handler

- Preserve scalar GPRs ttmp[4:11] and ttmp13
- Add single step exception during context save workaround
- Remove incorrect PC adjustment during context save

Signed-off-by: Jay Cornwall <jay.cornwall@amd.com>
Reviewed-by: Yong Zhao <Yong.Zhao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>