dma-buf: heaps: system: Remove global variable

The system heap is storing its struct dma_heap pointer in a global
variable but isn't using it anywhere.

Let's move the global variable into system_heap_create() to make it
local.

Signed-off-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250407-dma-buf-ecc-heap-v3-1-97cdd36a5f29@kernel.org


Signed-off-by: Christian König <christian.koenig@amd.com>