drm/amd/display: Clear update flags at end of flip

Clear update flags so the next flip does not have any redundant
programming (if a subsequent flip does not have a stream or plane
update, the update flags are not cleared).

Fixes: 0baae624 ("drm/amd/display: Refactor fast update to use new HWSS build sequence")
Reviewed-by: Jun Lei <jun.lei@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>