drm/amd/display: Use memset to initialize variable in wait_for_training_aux_rd_interval

Clang warns:

drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_link_dp.c:50:57:
warning: suggest braces around initialization of subobject
[-Wmissing-braces]
        union training_aux_rd_interval training_rd_interval = {0};
                                                               ^
                                                               {}
1 warning generated.

Previous efforts to fix this type of warning by adding or removing
braces have been met with some pushback in favor of using memset [1][2].
Do that here, mirroring commit 05794eff ("drm/amdgpu/gmc: fix
compiler errors [-Werror,-Wmissing-braces] (V2)") in this tree.

[1]: https://lore.kernel.org/lkml/022e41c0-8465-dc7a-a45c-64187ecd9684@amd.com/
[2]: https://lore.kernel.org/lkml/20181128.215241.702406654469517539.davem@davemloft.net/



Fixes: 3cec4176 ("drm/amd/display: Fix use of uninitialized union")
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>