drm/i915/ttm: pass along the I915_BO_ALLOC_CONTIGUOUS

Currently we just ignore the I915_BO_ALLOC_CONTIGUOUS flag, which is
fine since everything is already contiguous with the ttm range manager.
However in the next patch we want to switch over to the ttm buddy
manager, where allocations are by default not contiguous.

v2(Thomas):
    - Forward ALLOC_CONTIG for all regions

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210616152501.394518-4-matthew.auld@intel.com