drm/amdgpu/display: add helper functions to get/set backlight (v2)

And cache the value.  These can be used by the backlight callbacks
and modesetting functions.

v2: rebase on latest backlight changes.

Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1337
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -436,6 +436,13 @@ struct amdgpu_display_manager {
 	 */
 	struct list_head da_list;
 	struct completion dmub_aux_transfer_done;
+
+	/**
+	 * @brightness:
+	 *
+	 * cached backlight values.
+	 */
+	u32 brightness[AMDGPU_DM_MAX_NUM_EDP];
 };
 
 enum dsc_clock_force_state {