[Why]

For DCN hardware, the crtc_high_irq handler is assigned to the vstartup
interrupt. This is different from DCE, which has it assigned to vblank
start.

We'd like to send vblank and user events at vstartup because:

* It happens close enough to vupdate - the point of no return for HW.

* It is programmed as lines relative to vblank end - i.e. it is not in
  the variable portion when VRR is enabled. We should signal user
  events here.

* The pflip interrupt responsible for sending user events today only
  fires if the DCH HUBP component is not clock gated. In situations
  where planes are disabled - but the CRTC is enabled - user events won't
  be sent out, leading to flip done timeouts.

Consequently, this makes vupdate on DCN hardware redundant. It will be
removed in the next change.

[How]

Add a DCN-specific crtc_high_irq handler, and hook it to the VStartup
signal. Inside the DCN handler, we send off user events if the pflip
handler hasn't already done so.
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>