drm/panfrost: Fix GEM handle creation ref-counting

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

drm/panfrost: Fix GEM handle creation ref-counting

panfrost_gem_create_with_handle() previously returned a BO but with the

only reference being from the handle, which user space could in theory

guess and release, causing a use-after-free. Additionally if the call to

panfrost_gem_mapping_get() in panfrost_ioctl_create_bo() failed then

a(nother) reference on the BO was dropped.

The _create_with_handle() is a problematic pattern, so ditch it and

instead create the handle in panfrost_ioctl_create_bo(). If the call to

panfrost_gem_mapping_get() fails then this means that user space has

indeed gone behind our back and freed the handle. In which case just

return an error code.