drm/amd/display: Fix warning observed in mode change on Vega

[Why]
DOUBLE_BUFFER_EN bit is getting cleared before enable blanking.
That leads to CRTC_BLANK_DATA_EN is getting updated immediately.

[How]
Get DOUBLE_BUFFER_EN bit set, the same as DCE110.

Signed-off-by: Jerry (Fangzhi) Zuo <Jerry.Zuo@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>