drm/amd/display: wait vblank when stream enabled and update dpp clock

[Why]
When boot into OS, seamless boot device won't blank stream.
Driver update dpp clock when scanline position in vactive will show
garbage on screen.

[How]
Wait for vblank for seamless boot edp display when driver update dpp clock.
The apply seamless boot flag will be clear when OS call SetVisibility on.
Therefore we only wait for vblank once after boot into OS.

Signed-off-by: Lewis Huang <Lewis.Huang@amd.com>
Acked-by: Bindu Ramamurthy <bindur12@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>