drm/amd/display: change global buffer to local buffer

[Why]
Multi-adapter calculate regamma table at the same time.
Two thread used the same global variable cause race
condition.

[How]
Change global buffer to local buffer

Signed-off-by: Lewis Huang <Lewis.Huang@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>