drm/amd/display: Remove unused field

[why & how]
Remove force_backlight_start_level since it is never used.

Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -306,8 +306,6 @@ struct dc_bug_wa {
 		uint8_t dcfclk_ds: 1;
 	} clock_update_disable_mask;
 	bool skip_psr_ips_crtc_disable;
-	//Customer Specific WAs
-	uint32_t force_backlight_start_level;
 };
 struct dc_dcc_surface_param {
 	struct dc_size surface_size;