drm/amd/display: Remove unused *vbios_smu_set_dprefclk

rn_vbios_smu_set_dprefclk() was added in 2019 by
commit 4edb6fc9 ("drm/amd/display: Add Renoir clock manager")
rv1_vbios_smu_set_dprefclk() was also added in 2019 by
commit dc88b4a6 ("drm/amd/display: make clk mgr soc specific")

neither have been used.

Remove them.

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