drm/amd/display: Rename DCN config to FP

[Why & How]
The only reason we have the DCN config is for
floating point support. Rename it to make that
clear and (hopefully) avoid misuse of the config
in the future.

Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>