panfrost: Allocate RAM backing of shared memory

panfrost: Allocate RAM backing of shared memory

Unlike other GPUs, Mali does not have dedicated shared memory for
compute workloads. Instead, we allocate shared memory (backed to RAM),
and the general memory access functions have modes to access shared
memory (essentially, think of these modes as adding this allocates base
+ workgroupid * stride  in harder). So let's allocate enough memory
based on the shared_size parameter and supply it.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3835>