drm/amd/display: fix pflip irq registor for raven

The pflip registor index was incorrect, this was overriding the macro
and caused a dummy irq call.

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>