drm/amd/display: Initialize DSC PPS variables to 0

For DSC MST, sometimes monitors would break out
in full-screen static. The issue traced back to the
PPS generation code, where these variables were being used
uninitialized and were picking up garbage.

memset to 0 to avoid this

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: David Francis <David.Francis@amd.com>
Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>