drm/amdgpu: fix one warning message

In function 'amdgpu_uvd_cs_pass2':
warning: 'min_ctx_size' may be used uninitialized in this function
  buf_sizes[0x4] = min_ctx_size;
                 ^
note: 'min_ctx_size' was declared here
  unsigned image_size, tmp, min_dpb_size, num_dpb_buffer, min_ctx_size;
                                                          ^
Signed-off-by: Jammy Zhou <Jammy.Zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Cc: stable@vger.kernel.org