dsputil: Make square put/avg_pixels functions local to h264qpel

Put a copy of the 8bit functions only in dsputil, where they are
used for some other things (e.g. mpeg4qpel, mspel, cavsqpel).

Signed-off-by: Martin Storsjö <martin@martin.st>
h264: Clear the mb members via memset instead of using dsputil

Signed-off-by: Martin Storsjö <martin@martin.st>
h264: Integrate clear_blocks calls with IDCT

The non-intra-pcm branch in hl_decode_mb (simple, 8bpp) goes from 700
to 672 cycles, and the complete loop of decode_mb_cabac and hl_decode_mb
(in the decode_slice loop) goes from 1759 to 1733 cycles on the clip
tested (cathedral), i.e. almost 30 cycles per mb faster.

Signed-off-by: Martin Storsjö <martin@martin.st>
h264: Only initialize dsputil if error resilience is enabled

It is only used for error resilience. This allows building the
h264 decoder without dsputil, if error resilience is disabled.

Signed-off-by: Martin Storsjö <martin@martin.st>
dsputil: Make dsputil selectable

Signed-off-by: Martin Storsjö <martin@martin.st>