drm: amdgpu: pm: Mark vangogh_clk_dpm_is_enabled() as static

kernel test robot throws below warnings ->

drivers/gpu/drm/amd/amdgpu/../pm/swsmu/smu11/vangogh_ppt.c:594:6:
warning: no previous prototype for 'vangogh_clk_dpm_is_enabled'
[-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../pm/swsmu/smu11/vangogh_ppt.c:594:6:
warning: no previous prototype for function 'vangogh_clk_dpm_is_enabled'
[-Wmissing-prototypes]

Mark vangogh_clk_dpm_is_enabled() as static.

Reviewed-by: Huang Rui <ray.huang@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>