drm/amd/display: fix stuck test pattern on right half of display

[why]
With visual confirm enabled, displays where ODM combine is enabled
has a test pattern stuck on the right half of the display even
though the display is unblanked.

[how]
Add a condition to not show the colour ramp test pattern when the
display is unblanked.

Signed-off-by: Zi Yu Liao <ziyu.liao@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>