drm/amdkfd: Preserve wave state after instruction fetch MEM_VIOL

If instruction fetch fails the wave cannot be halted and returned to
the shader without raising MEM_VIOL again. Currently the wave is
terminated if this occurs, but this loses information about the cause
of the fault. The debugger would prefer the faulting wave state to be
context-saved.

Poll inside the trap handler until TRAPSTS.SAVECTX indicates context
save is ready. Exit the poll loop and complete the remainder of the
exception handler, then return to the shader. The next instruction
fetch will be from the trap handler and not the faulting PC. Context
save will then deschedule the wave and save its state.

Signed-off-by: Jay Cornwall <Jay.Cornwall@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>