drm/amd/display: Remove unused hubbub1_toggle_watermark_change_req

hubbub1_toggle_watermark_change_req() last use was removed in 2017 by
commit b8fce2c9 ("drm/amd/display: Optimize programming front end")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>