drm/amdgpu/dce11: update async flip update time

Use UPDATE_IMMEDIATE (update on next data request boundary) rather
than UPDATE_H_RETRACE (update on next line boundary).  The data
request boundary is less than a scanline, so it update will happen
sooner.

Cc: Vitaly Prosyak <vitaly.prosyak@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>