drm/amd/display: set variable dccg314_init storage-class-specifier to static

smatch reports
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn314/dcn314_dccg.c:277:6: warning: symbol
  'dccg314_init' was not declared. Should it be static?

This variable is 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>