drm/msm: Avoid NULL dereference in msm_disp_state_print_regs()

In the Linux kernel, the following vulnerability has been resolved: drm/msm: Avoid NULL dereference in msm_disp_state_print_regs() If the allocation in msm_disp_state_dump_regs() failed then `block->state` can be NULL. The msm_disp_state_print_regs() function _does_ have code to try to handle it with: if (*reg) dump_addr = *reg; ...but since "dump_addr" is initialized to NULL the above is actually a noop. The code then goes on to dereference `dump_addr`. Make the function print "Registers not stored" when it sees a NULL to solve this. Since we're touching the code, fix msm_disp_state_print_regs() not to pointlessly take a double-pointer and properly mark the pointer as `const`. Patchwork: https://patchwork.freedesktop.org/patch/619657/