drm/amd/display: Reduce stack usage

Reduce stack usage by moving an unnecessary structure copy to a pointer.

Reviewed-by: Joshua Aberback <Joshua.Aberback@amd.com>
Acked-by: Pavle Kotarac <Pavle.Kotarac@amd.com>
Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>