drm/amd/display: Initialize stream_update with memset

The brace initialization used here generates warnings on some
compilers. For example, on GCC 4.9:

[...] In function ‘dm_determine_update_type_for_commit’:
[...] error: missing braces around initializer [-Werror=missing-braces]
   struct dc_stream_update stream_update = { 0 };
          ^

Use memset to make this more portable.

v2: Specify the compiler / diagnostic in the commit message (Paul)

Cc: Sun peng Li <Sunpeng.Li@amd.com>
Cc: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>