drm/amdgpu: mmVM_L2_CNTL3 register not initialized correctly

[ Upstream commit b8983d42

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The mmVM_L2_CNTL3 register is not assigned an initial value

Signed-off-by: Qu Huang <jinsdb@126.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>