drm/amd/display: based on flag reset z10 function pointer

[Why & How]
Per hardware requirements, add a flag to control
z10 enable/disable.

Reviewed-by: Sung joon Kim <Sungjoon.Kim@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>