drm/amd/display: enable pipe power gating by default

[why]
ASIC requirement.

[how]
Make disable_*_power_gate to false.

Signed-off-by: Sung Joon Kim <sungjoon.kim@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>