drm/amd/display: program DPG_OFFSET_SEGMENT for odm_pipe

[why]
When test pattern is enabled with ODM combine, test pattern is generated
by piecing multiple DPGs image together.  The current code will program
all DPGs with horizontal offset of 0. This will cause all DPGs to output
the beginning of the pattern. Instead each DPG should program a
horizontal offset of its x position to form a continous pattern when
pieced together.

Signed-off-by: Wenjing Liu <Wenjing.Liu@amd.com>
Reviewed-by: Nikola Cornij <Nikola.Cornij@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>