drm/amdgpu: declare smuio v13_0 callbacks as static

fix -Wmissing-protoypes warning
Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Likun Gao <Likun.Gao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/smuio_v13_0.c

@@ -37,7 +37,7 @@ static u32 smuio_v13_0_get_rom_data_offset(struct amdgpu_device *adev)
 	return SOC15_REG_OFFSET(SMUIO, 0, regROM_DATA);
 }
 
-void smuio_v13_0_update_rom_clock_gating(struct amdgpu_device *adev, bool enable)
+static void smuio_v13_0_update_rom_clock_gating(struct amdgpu_device *adev, bool enable)
 {
 	u32 def, data;
 
@@ -58,7 +58,7 @@ void smuio_v13_0_update_rom_clock_gating(struct amdgpu_device *adev, bool enable
 		WREG32_SOC15(SMUIO, 0, regCGTT_ROM_CLK_CTRL0, data);
 }
 
-void smuio_v13_0_get_clock_gating_state(struct amdgpu_device *adev, u32 *flags)
+static void smuio_v13_0_get_clock_gating_state(struct amdgpu_device *adev, u32 *flags)
 {
 	u32 data;