drm/amd/display: Introduce power module on Linux

drm/amd/display: Introduce power module on Linux

[Why]

Other OS supported by DC uses the power module to manage panel power
features such as backlight and self-refresh. It contains enhancements
on top what amdgpu_dm is doing today that can benefit power.

[How]

Introduce the power module. It's currently not being used anywhere, a
future change will incorporate it into amdgpu_dm.

Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Signed-off-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Chenyu Chen <chen-yu.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>