[Why]
pflip interrupt would not be enabled promptly if a pipe is disabled
and re-enabled, causing flip_done timeout error during DP
compliance tests

[How]
Enable pflip interrupt upon pipe enablement
Tested-by: NDaniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: NQingqing Zhuo <qingqing.zhuo@amd.com>
Reviewed-by: NNicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: NEryk Brol <eryk.brol@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>