drm/msm/a6xx: Fix kvzalloc vs state_kcalloc usage

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

drm/msm/a6xx: Fix kvzalloc vs state_kcalloc usage

adreno_show_object() is a trap! It will re-allocate the pointer it is
passed on first call, when the data is ascii85 encoded, using kvmalloc/
kvfree(). Which means the data *passed* to it must be kvmalloc'd, ie.
we cannot use the state_kcalloc() helper.

This partially reverts commit ec8f1813bf8d ("drm/msm/a6xx: Replace
kcalloc() with kvzalloc()"), but adds the missing kvfree() to fix the
memory leak that was present previously. And adds a warning comment.

Patchwork: https://patchwork.freedesktop.org/patch/507014/