drm/amdkfd: Clear the VALU exception state in the trap handler

The trap handler could be entered with pending VALU exceptions, so
clear the exception state before issuing vector instructions.

Reviewed-by: Jay Cornwall <jay.cornwall@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Laurent Morichetti <laurent.morichetti@amd.com>
Tested-by: Lancelot Six <lancelot.six@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>