drm/amd/display: set variables aperture_default_system and context0_default_system storage-class-specifier to static

smatch reports
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn10/dcn10_hubp.c:758:10: warning: symbol
  'aperture_default_system' was not declared. Should it be static?
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn10/dcn10_hubp.c:759:10: warning: symbol
  'context0_default_system' was not declared. Should it be static?

These variables are only used in one file so should be static.
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>