drm/amdgpu: Initialize shadow to false in gfx_v9_0_rlcg_wreg

clang warns:

drivers/gpu/drm/amd/amdgpu/gfx_v9_0.c:754:6: warning: variable 'shadow'
is used uninitialized whenever 'if' condition is
false [-Wsometimes-uninitialized]
        if (offset == grbm_cntl || offset == grbm_idx)
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/amd/amdgpu/gfx_v9_0.c:757:6: note: uninitialized use
occurs here
        if (shadow) {
            ^~~~~~
drivers/gpu/drm/amd/amdgpu/gfx_v9_0.c:754:2: note: remove the 'if' if
its condition is always true
        if (offset == grbm_cntl || offset == grbm_idx)
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/amd/amdgpu/gfx_v9_0.c:738:13: note: initialize the
variable 'shadow' to silence this warning
        bool shadow;
                   ^
                    = 0
1 warning generated.

It is not wrong so initialize shadow to false to ensure shadow is always
used initialized.

Fixes: 2e0cc4d ("drm/amdgpu: revise RLCG access path")
Link: ClangBuiltLinux#936
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>

drivers/gpu/drm/amd/amdgpu/gfx_v9_0.c

@@ -735,7 +735,7 @@ void gfx_v9_0_rlcg_wreg(struct amdgpu_device *adev, u32 offset, u32 v)
 	static void *spare_int;
 	static uint32_t grbm_cntl;
 	static uint32_t grbm_idx;
-	bool shadow;
+	bool shadow = false;
 
 	scratch_reg0 = adev->rmmio + (adev->reg_offset[GC_HWIP][0][mmSCRATCH_REG0_BASE_IDX] + mmSCRATCH_REG0)*4;
 	scratch_reg1 = adev->rmmio + (adev->reg_offset[GC_HWIP][0][mmSCRATCH_REG1_BASE_IDX] + mmSCRATCH_REG1)*4;