drm/amdgpu: Fix uninitialized variable use warning

Fix uninitialized variable use
warning: variable 'reg_access_ctrl' is uninitialized when used here [-Wuninitialized]
     scratch_reg0 = (void __iomem *)adev->rmmio + 4 * reg_access_ctrl->scratch_reg0;

Fixes: 5d447e29 ("drm/amdgpu: add helper for rlcg indirect reg access")
Reported-by: kernel test robot <yujie.liu@intel.com>
Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>