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>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.h

@@ -46,6 +46,7 @@
 
 #define AMDGPU_DM_MAX_CRTC 6
 
+#define AMDGPU_DM_MAX_NUM_EDP 2
 /*
 #include "include/amdgpu_dal_power_if.h"
 #include "amdgpu_dm_irq.h"
@@ -354,7 +355,10 @@ struct amdgpu_display_manager {
 
 	struct backlight_device *backlight_dev;
 
-	const struct dc_link *backlight_link;
+	const struct dc_link *backlight_link[AMDGPU_DM_MAX_NUM_EDP];
+
+	uint8_t num_of_edps;
+
 	struct amdgpu_dm_backlight_caps backlight_caps;
 
 	struct mod_freesync *freesync_module;