drm/amdgpu: Add preferred mode in modeset when freesync video mode's enabled.

[Why]
With kernel module parameter "freesync_video" is enabled, if the mode
is changed to preferred mode(the mode with highest rate), then Freesync
fails because the preferred mode is treated as one of freesync video
mode, and then be configurated as freesync video mode(fixed refresh
rate).

[How]
Skip freesync fixed rate configurating when modeset to preferred mode.

Signed-off-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org