drm/amd/display: Initialize necessary uninitialized variables

This commit initializes uninitialized variables.
For some compilers uninitialized variable warnings are treated as Error.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-by: Alan Liu <haoping.liu@amd.com>
Signed-off-by: Meera Patel <meera.patel@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>