drm/amdgpu/display: don't assert in set backlight function

It just spams the logs.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_link.c

@@ -2614,7 +2614,6 @@ bool dc_link_set_backlight_level(const struct dc_link *link,
 			if (pipe_ctx->plane_state == NULL)
 				frame_ramp = 0;
 		} else {
-			ASSERT(false);
 			return false;
 		}