drm/amd/display: Fix uninitialized variables in DC

This fixes 29 UNINIT issues reported by Coverity.

Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>