drm/amd/display: Fix a compilation failure on PowerPC caused by FPU code

We got a report from Stephen/Michael that the PowerPC build was failing
with the following error:

ld: drivers/gpu/drm/amd/display/dc/dml/display_mode_lib.o uses hard float, drivers/gpu/drm/amd/display/dc/dcn30/dcn30_optc.o uses soft float
ld: failed to merge target specific data of file drivers/gpu/drm/amd/display/dc/dcn30/dcn30_optc.o

This error happened because of the function optc3_set_vrr_m_const. This
function expects a double as a parameter in a code that is not allowed
to have FPU operations. After further investigation, it became clear
that optc3_set_vrr_m_const was never invoked, so we can safely drop this
function and fix the ld issue.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Melissa Wen <mwen@igalia.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>