drm/ioc32: stop speculation on the drm_compat_ioctl path

In the Linux kernel, the following vulnerability has been resolved:

drm/ioc32: stop speculation on the drm_compat_ioctl path

The drm compat ioctl path takes a user controlled pointer, and then

dereferences it into a table of function pointers, the signature method

of spectre problems. Fix this up by calling array_index_nospec() on the

index to the function pointer list.