kgsl: fix genalloc BUG when detroying pagetable.

kgsl allocs some memory (in map_global), but never free it.
Fixes that.
Only happens when on GPUMMU ..
Revert "genalloc: Temp workaround. KGSL when using in GPUMMU does some weird things in gen_pool, that crashes everything. This avoid kernel to panic."

This reverts commit f2c9bd2b6f45bbe0c4eaaa217bf848c18637e13f.