drm/amd/display: Change dither policy for 10bpc to round

We use spatial dither by default for all output bpc (6/8/10). While it
makes some sense for FP16, for ARGB2101010 surfaces it makes little
sense as even if we skip color pipeline to preserve bit accuracy,
spatial dither adds random noise so a few percent pixels are 1 bit off.
This commit chages the 10bpc out dither policy to rounding.

Also, in Polaris/Vega times, policy used to be round for 10bpc out; it
looks like it got inadvertently changed for Navi. Difference is only
detectable with capture cards.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Reviewed-by: Anthony Koo <anthony.koo@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Krunoslav Kovac <krunoslav.kovac@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>