intel: Add a bo_alloc function for tiled BOs.

intel: Add a bo_alloc function for tiled BOs.

This simplifies driver code in handling object allocation, and also gives us
an opportunity to possibly cache tiled buffers if it turns out to be a win.

[anholt: This is chopped out of the execbuf2 patch, as it seems to be useful
separately and cleans up the execbuf2 changes to be more obvious]