drm/amd/display: Remove unused globals FORCE_RATE and FORCE_LANE_COUNT

sparse reports
drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_link_dp.c:3885:6: warning: symbol 'FORCE_RATE' was not declared. Should it be static?
drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_link_dp.c:3886:10: warning: symbol 'FORCE_LANE_COUNT' was not declared. Should it be static?

Neither of thse variables is used in dc_link_dp.c.  Reviewing the commit listed in
the fixes tag shows neither was used in the original patch.  So remove them.

Fixes: 265280b9 ("drm/amd/display: add CLKMGR changes for DCN32/321")
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>