drm/amd/display: change several dcn30 variables storage-class-specifier to static

drm/amd/display: change several dcn30 variables storage-class-specifier to static

smatch reports these similar problems in dcn30
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn30/dcn30_dwb.c:223:25:
  warning: symbol 'dcn30_dwbc_funcs' was not declared. Should it be static?
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn30/dcn30_mmhubbub.c:214:28:
  warning: symbol 'dcn30_mmhubbub_funcs' was not declared. Should it be static?
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn30/dcn30_mpc.c:1402:24:
  warning: symbol 'dcn30_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>