h264: Add add_pixels4/8() to h264dsp, and remove add_pixels4 from dsputil

These functions are mostly H264-specific (the only other user I can
spot is bink), and this allows us to special-case some functionality
for H264. Also remove the 16-bit-coeff with >8bpp versions (unused)
and merge the duplicate 32-bit-coeff for >8bpp (identical).

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