drm/amd/display: Add missing pflip irq for dcn2.0

If we have more than 4 displays we will run
into dummy irq calls or flip timout issues.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org