drm/amd: Remove freesync video mode amdgpu parameter

[Why&How]
Freesync Video mode was enabled by default. Hence no need for the module
parameter, so remove it completely

Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Reviewed-by: Stylon Wang <stylon.wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>