v3dv: don't leak cache key
What does this MR do and why?
v3dv: don't leak cache key
The hashtable stores a hash generated from the key, so there is no need
to duplicate the key, as otherwise it is a leak.
Fixes: 4f26303dbb1 ("v3dv: add debug option to disable custom pipeline caches for meta operations")
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>