i915: Allocate the sys_buffer using _mesa_align_malloc

i915: Allocate the sys_buffer using _mesa_align_malloc

Though it won't matter on Linux, use _mesa_align_free to release it.
Since i965 doesn't have sys_buffer, I overlooked this in the
GL_ARB_map_buffer_alignment work a few months ago.  Fixes i915 (and
presumably i830) regressions in ARB_map_buffer_range tests and the
failure in arb_map_buffer_alignment-sanity_test.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=74960
Cc: "10.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>