drm/amd/display: make variables static

As "dcn3_1_soc", "dcn3_15_soc", and "dcn3_16_soc" are not used outside
of their corresponding "dcn3*_fpu.c", make them static and remove their
extern declaration.

Fixes: 26f4712a ("drm/amd/display: move FPU related code from dcn31 to dml/dcn31 folder")
Fixes: fa896297 ("drm/amd/display: move FPU related code from dcn315 to dml/dcn31 folder")
Fixes: 3f8951cc

 ("drm/amd/display: move FPU related code from dcn316 to dml/dcn31 folder")
Signed-off-by: Magali Lemes <magalilemes00@gmail.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Reviewed-by: Maíra Canal <mairacanal@riseup.net>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>