drm/amd/display: Fix recout calculation for stereo side-by-side

[why & how]
The recout x offset was incorrect which led to
wrong viewport calculation. For stereo
side-by-side case, the slice index should be
0 for both split pipes.

Reviewed-by: Dmytro Laktyushkin <dmytro.laktyushkin@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Sung Joon Kim <sungjoon.kim@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>