drm/amd/display: Add center mode for integer scaling in DC

[why]
We want to use maximum space on display to show source

[how]
For Centered Mode: Replicate source as many times as possible to use
maximum of display active space add borders.

Signed-off-by: Reza Amini <Reza.Amini@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>