Use OpenMP threads in bm_loop_interp_mdisps

Gives approx 2x speedup on my laptop on such operations
as mesh subdivision in edit mode. Desktops with fancier
CPUs could benefit even more.

Thanks Campbell for review!

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57375 954f8c5b-7b00-dc11-b283-0030488c597c