i965: Ensure that intel_bufferobj_map_range meets alignment guarantees

i965: Ensure that intel_bufferobj_map_range meets alignment guarantees

No piglit regressions on IVB.

With minor tweaks to the arb_map_buffer_alignment-map-invalidate-range
test (disable the extension check, set alignment to 64 instead of
querying), the i965 driver would fail the test without this patch (as
predicted by Eric).  With this patch, it passes.

v2: Remove MAX2(64, ...).  Suggested by Ken.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Cc: Siavash Eliasi <siavashserver@gmail.com>