drm/amd/display: Use drm_fb_helper_poll_changed()

This driver can use drm_fb_helper_output_poll_changed() as its
.output_poll_changed callback.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>