drm/amd/display: Add backlight pwm debugfs

[Why]
ABM enablement testing can be automated if a way of reading
target and current hardware backlight is available

[How]
Expand debugfs interface with two new entries.
Hook directly into dc interface.  Units are as
a fraction of 0x1000 = 100%

Use the built-in amdgpu function for creating
read-only debugfs files

Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>