drm/amd/display: Remove unused optc3_fpu_set_vrr_m_const

The last use of optc3_fpu_set_vrr_m_const() was removed in 2022's
commit 64f99159 ("drm/amd/display: Fix a compilation failure on PowerPC
caused by FPU code")
which removed the only caller (with a similar) name.

Remove it.

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