dsputil: create 16/32-bit dctcoef versions of some functions

High bitdepth H.264 needs 32-bit transform coefficients, whereas
dnxhd does not.  This creates a conflict with the templated
functions operating on DCTELEM data.  This patch adds a field
allowing the caller to choose the element size in dsputil_init()
and adds the required functions.

Signed-off-by: Mans Rullgard <mans@mansr.com>