drm/amd/display: change several dcn20 variables storage-class-specifier to static

smatch reports these similar problems in dcn20
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn20/dcn20_dsc.c:53:24:
  warning: symbol 'dcn20_dsc_funcs' was not declared. Should it be static?
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn20/dcn20_dwb.c:304:25:
  warning: symbol 'dcn20_dwbc_funcs' was not declared. Should it be static?
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn20/dcn20_mmhubbub.c:300:28:
  warning: symbol 'dcn20_mmhubbub_funcs' was not declared. Should it be static?
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn20/dcn20_mpc.c:545:24:
  warning: symbol 'dcn20_mpc_funcs' was not declared. Should it be static?

All of these are only used in their definition file, so they should be static

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>