drm/amd/display: Remove unused get_clock_requirements_for_state

get_clock_requirements_for_state() was added in 2018 by
commit 8ab2180f ("drm/amd/display: Add function to fetch clock
requirements")
but never used.

Remove it.

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