drm/amdgpu: mark symbols static where possible

We get 2 warnings when building kernel with W=1:
drivers/gpu/drm/amd/amdgpu/si.c:908:5: warning: no previous prototype for 'si_pciep_rreg' [-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/si.c:921:6: warning: no previous prototype for 'si_pciep_wreg' [-Wmissing-prototypes]

In fact, both functions are only used in the file in which they are
declared and don't need a declaration, but can be made static.
So this patch marks these functions with 'static'.
Acked-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -906,7 +906,7 @@ static void si_pcie_wreg(struct amdgpu_device *adev, u32 reg, u32 v)
 	spin_unlock_irqrestore(&adev->pcie_idx_lock, flags);
 }
 
-u32 si_pciep_rreg(struct amdgpu_device *adev, u32 reg)
+static u32 si_pciep_rreg(struct amdgpu_device *adev, u32 reg)
 {
 	unsigned long flags;
 	u32 r;
@@ -919,7 +919,7 @@ u32 si_pciep_rreg(struct amdgpu_device *adev, u32 reg)
 	return r;
 }
 
-void si_pciep_wreg(struct amdgpu_device *adev, u32 reg, u32 v)
+static void si_pciep_wreg(struct amdgpu_device *adev, u32 reg, u32 v)
 {
 	unsigned long flags;