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.

Fixes: ec8f1813bf8d ("drm/msm/a6xx: Replace kcalloc() with kvzalloc()")
Closes: https://gitlab.freedesktop.org/drm/msm/-/issues/20
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Akhil P Oommen <quic_akhilpo@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/507014/
Link: https://lore.kernel.org/r/20221013225520.371226-2-robdclark@gmail.com