drm/i915/lmem: Limit block size to 4G

Block sizes are only limited by the largest power-of-two that will fit
in the region size, but to construct an object we also require feeding
it into an sg list, where the upper limit of the sg entry is at most
UINT_MAX. Therefore to prevent issues with allocating blocks that are
too large, add the flag I915_ALLOC_MAX_SEGMENT_SIZE which should limit
block sizes to the i915_sg_segment_size().

v2: (matt)
  - query the max segment.
  - prefer flag to limit block size to 4G, since it's best not to assume
    the user will feed the blocks into an sg list.
  - simple selftest so we don't have to guess.

Cc: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: CQ Tang <cq.tang@intel.com>
Signed-off-by: Venkata Sandeep Dhanalakota <venkata.s.dhanalakota@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20201130134721.54457-1-matthew.auld@intel.com