gpu: ion: Fix a contiguous system heap crash

map_kernel and unmap_kernel in kmalloc_ops should support
a contiguous heap. The existing functions
ion_system_heap_map_kernel and ion_system_heap_unmap_kernel
are geared towards discontiguous memory (supported via
vmalloc_ops) and crash while handling contiguous heaps. This
patch adds two new functions specific to contiguous system
heaps to initialize kmalloc_ops.

Change-Id: I48d30ca01c5a0b71af4e1e640c79a920a298aedb
Acked-by: Jennifer L. Zenner <jzenner@qti.qualcomm.com>
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>