drm/amd/display: adjust visual confirm dpp height offset to be 1/240 of v addressable

[why]
For timing with large v addressable visual confirm is just too small. It is difficult
to tell visually which DPP we are using. On the other hand with timing with small
v addressable visual confirm is too large and covers the UI area.

[how]
We calculate visual confirm dpp height offset based on v addressable so it stays
relatively the same height i.e. 1/240 verticle portion of the screen.

Reviewed-by: Jun Lei <jun.lei@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>