drm/i915/region: support contiguous allocations

Some kernel internal objects may need to be allocated as a contiguous
block, also thinking ahead the various kernel io_mapping interfaces seem
to expect it, although this is purely a limitation in the kernel
API...so perhaps something to be improved.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Cc: Michael J Ruhl <michael.j.ruhl@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/20191008160116.18379-3-matthew.auld@intel.com