drm/radeon: Add support for programming the FMT blocks

The FMT blocks control how data is sent from the backend
of the display pipe to to monitor.  Proper set up of the
FMT blocks are required for 30bpp formats.  Additionally,
dithering can be enabled on for better display with 18 and
24bpp displays.  The exception is LVDS/eDP which atom
takes care of in the SelectCRTC_Source table.  For now
just enable truncation until we test dithering more.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>