drm/amd/display: Enable double buffer for OTG_BLANK

[Why]
Currently if seamless boot is enabled, we will skip double buffer enable
for OTG_BLANK. However, we need the double buffer enable in order to
block global sync signals when OTG becomes blanked (for PSR). Blocking
global sync signals prevent pipe from requesting data.

[How]
Move tg_init before seamless boot check.

Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>