Refactored idct routines and headers

Refactored idct routines and headers

This change is made in preparation for a
subsequent patch which adds acceleration
for the highbitdepth transform functions.

The highbitdepth transform functions attempt
to use 16/32bit sse instructions where possible,
but fallback to using the C implementations if
potential overflow is detected.  For this reason
the dct routines are made global so they can be
called from the acceleration functions in the
subsequent patch.

Change-Id: Ia921f191bf6936ccba4f13e8461624b120c1f665