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/