drm/amd/display: multi-eDP backlight support

[why]
Currently the assumption is that we are using a single eDP
connector so there will only be one backlight object. Need changes
to allow brightness update and reading for multiple eDP connectors.

[how]
- register a single device
- turn backlight link from a pointer to an array of pointers
- update brightness of all eDP links at the same time when request
is registered
- read brightness level only of the primary eDP panel
- turn current_backlight_pwm and targer_backlight_pwm debugfs enteries
into per connector enteries.

Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Wayne Lin <waynelin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>