kernel: drm/amdgpu: Fix a null pointer access when the smc_rreg pointer is NULL

In the Linux kernel, the following vulnerability has been resolved: drm/amdgpu: Fix a null pointer access when the smcrreg pointer is NULL In certain types of chips, such as VEGA20, reading the amdgpuregssmc file could result in an abnormal null pointer access when the smcrreg pointer is NULL...