Define macro AMDGPU_MAX_SG_SEGMENT_SIZE 2GB, because struct scatterlist
length is unsigned int, and some users of it cast to a signed int, so
every segment of sg table is limited to size 2GB maximum.

For contiguous VRAM allocation, don't limit the max buddy block size in
order to get contiguous VRAM memory. To workaround the sg table segment
size limit, allocate multiple segments if contiguous size is bigger than
AMDGPU_MAX_SG_SEGMENT_SIZE.
Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>