This removes the last use of the ff_put/add_pixels_clamped global
function pointers, and as such they are removed.

This patch has a negative effect on performance on MIPS, since there's
a SIMD-optimized put/add_pixels_clamped, but no xvid or jrev. From a
code maintenance point of view, that is probably acceptable.

Because the global function pointers are removed, this fixes the following
tsan warnings when running e.g. fate-dnxhr-parse:

WARNING: ThreadSanitizer: data race (pid=29917)
  Write of size 8 at 0x0000025b12d8 by thread T2 (mutexes: write M1543):
    #0 ff_idctdsp_init src/libavcodec/idctdsp.c:313 (ffmpeg+0x00000044b68e)
[..]
  Previous write of size 8 at 0x0000025b12d8 by thread T1 (mutexes: write M1541):
    #0 ff_idctdsp_init src/libavcodec/idctdsp.c:313 (ffmpeg+0x00000044b68e)