Fixes gcc '-Wunused-but-set-variable' warning:

rivers/gpu/drm/amd/amdgpu/../display/modules/freesync/freesync.c:
 In function mod_freesync_get_settings:
drivers/gpu/drm/amd/amdgpu/../display/modules/freesync/freesync.c:984:24:
 warning: variable core_freesync set but not used [-Wunused-but-set-variable]

It is not used since commit 98e6436d ("drm/amd/display: Refactor FreeSync module")
Reviewed-by: NHarry Wentland <harry.wentland@amd.com>
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>