drm/amdkfd: Fix lost single step exceptions in gfx9 trap handler

drm/amdkfd: Fix lost single step exceptions in gfx9 trap handler

If the trap is entered due to MODE.DEBUG_EN=1 and SAVECTX is raised
concurrently the handler cannot identify the source of the exception.
This causes the debugger to lose single step exception notification
when a context save request arrives at the same time.

When MODE.DEBUG_EN=1 and STATUS.HALT=0 (exception not already handled)
jump to the second-level trap handler upon entering the trap. The
second-level trap will set STATUS.HALT=1 and return to the shader.
If SAVECTX was raised then control flow will return to the trap, which
will then handle the context save request.

Cc: Tony Tye <tony.tye@amd.com>
Cc: Laurent Morichetti <laurent.morichetti@amd.com>
Cc: Qingchuan Shi <qingchuan.shi@amd.com>
Signed-off-by: Jay Cornwall <jay.cornwall@amd.com>
Reviewed-by: Laurent Morichetti <laurent.morichetti@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>