drm/amd/display: Enable pflip interrupt upon pipe enable

[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: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>