drm/amd/display: Move define to the proper header

[Why & How]
DCN3_16_MIN_COMPBUF_SIZE_KB is defined in the dcn316_resource.c file.
This header fit better in the dcn31_fpu.h together with similar defines.

Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>