drm/amd/display: Fix DML failures caused by doubled stereo viewport

[Why]
Side-by-side and Top-and-bottom stereo configurations fail DML mode
validation due to Viewport exceeded.

This is because we consider the planes as being pipe split in pipe
population so we end up doubling the viewport width, eg. from 4k to 8k.

[How]
These pipes technically aren't hsplit, so add a check for determining
whether is_hsplit should be set.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>