drm/amd/display: set variable dcn3_14_soc storage-class-specifier to static

smatch reports
drivers/gpu/drm/amd/amdgpu/../display/dc/dml/dcn314/dcn314_fpu.c:100:37: warning: symbol
  'dcn3_14_soc' 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>