drm/amdgpu: Reduce VA_RESERVED_BOTTOM to 64KB

The reservation is there to catch NULL pointer dereferences from the
GPU. Reduce the size to 64KB to make sure that shared virtual address
programming models can map all CPU-accessible virtual addresses for GPU
access. This is also the default for CPU virtual address mappings as
seen in /proc/sys/vm/mmap_min_addr.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>