drm/amd/display: Enable Freesync Video Mode by default

[Why&How]
Freesync Video Mode is a experimental feature previously,
and need to be enabled by kernel parameter. We enable it
by default with removing module paramterter in amdgpu_dm.

v2: squash the patches together

Signed-off-by: Solomon Chiu <solomon.chiu@amd.com>
Reviewed-by: Aurabindo Jayamohanan Pillai <Aurabindo.Pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>