drm/amdgpu: clear set_q_mode_offs when VM changed

[Why]
set_q_mode_offs don't get cleared after GPU reset, nexting SET_Q_MODE
packet to init shadow memory will be skiped, hence there has a page fault.

[How]
VM flush is needed after GPU reset, clear set_q_mode_offs when
emitting VM flush.

Fixes: 8bc75586 ("drm/amdgpu: workaround to avoid SET_Q_MODE packets v2")
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: ZhenGuo Yin <zhenguo.yin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org