i965/bufmgr: Add a new, simpler, bo_alloc_tiled

i965/bufmgr: Add a new, simpler, bo_alloc_tiled

ISL already has all of the complexity required to figure out the correct
surface pitch and size taking tile alignment into account.  When we get
a surface out of ISL, the pitch and size are already correct and using
brw_bo_alloc_tiled_2d doesn't actually gain us anything other than extra
asserts we have to do in order to ensure that the bufmgr code and ISL
agree.  This new helper doesn't try to be smart but just allocates the
BO you ask for and sets up the tiling.

Reviewed-by: Plamena Manolova <plamena.manolova@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>