drm/amd/display: zeromem mypipe heap struct before using it

[Why & How]
bug was caused when moving variable from stack to
heap because it was reusable and garbage was left
over, so we need to zero mem

Fixes: 7acc487a ("drm/amd/display: reduce stack size in dcn32 dml (v2)")
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Martin Leung <Martin.Leung@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org