drm/amd/display: Unify all scaling when Integer Scaling enabled

[why]
We want to guarantee integer ratio scaling for all scaling modes.

[how]
Treat centered, fullscreen, preserve aspect ratio the same: scale
the view as many times as possible, and fill in the rest with a black
border.

Signed-off-by: Reza Amini <Reza.Amini@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>