drm/amd/display: Add custom segmentation support

[WHY]
Add support for uneven segmentation size

[HOW]
Currently width + x_start is automatically calculated using
slice index + dividing stream into equally sized segments.
Add override variables for dst_rect width and x

Reviewed-by: Samson Tam <samson.tam@amd.com>
Signed-off-by: Brendan Steve Leder <BrendanSteve.Leder@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>