drm/amd/powerplay: set vega12 pre display configurations

Set num_displays to 0 and force uclk high as part of the mode
set sequence.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>