drm/amd/display: remove set but not used variable 'core_freesync'

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: Harry Wentland <harry.wentland@amd.com>
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>