drm/amd/display: Add emulated sink support for updating FS

[Why]
When forced modes are used during certain IGT tests,
without a real connector, dc_sink would be null when
standard modes are added by the driver. Calling the
function to update freesync capabilities at this
point will result in an error being printed

[How]
Use emulated sink when available. If both the normal
and emulated sink are not available, set all freesync
parameters to 0.

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <Daniel.Wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>