vf_eq: remove slow inline asm

Compiled with -O2, the C code runs just as far (or even slightly faster)
then the MMX inline asm.
vf_ilpack: remove inline asm

This makes it multiple times slower. However, the output format (packed
YUV) isn't handled efficiently by anything to begin with, and I have no
clue we even have this filter. I guess it's one of these filters which
find some use sometimes, but are not of higher importance, which
justifies removing the faster inline asm.
vf_noise: remove line asm

I didn't test the speed, but by default, this filter diverts to
libavfilter already. So this would help only if libavfilter is disabled,
or libavfilter doesn't have vf_noise (like on Libav). For these cases,
we still provide the (possibly but not necessarily) slower C
implementation of vf_noise.
vf_pullup: remove inline asm

No change in speed (or even slightly faster, though I tested with
progressive solid color video only), and normally we use libavformat's
vf_pullup anyway.
vf_divtc: remove inline asm

Becomes a bit slower (tested with progressive solid color video only),
but this filter is pretty obscure and I'm not even sure what it's useful
for.
vf_pp: use native libpostproc CPU detection
vf_pp: fix include statement
Remove CPU detection and inline asm handling

Not needed anymore. I'm not opposed to having asm, but inline asm is too
much of a pain, and it was planned long ago to eventually get rid fo all
inline asm uses.

For the note, the inline asm use that was removed with the previous
commits was almost worthless. It was confined to video filters, and most
video filtering is now done with libavfilter. Some mpv filters (like
vf_pullup) actually redirect to libavfilter if possible.

If asm is added in the future, it should happen in the form of external
files.
Fix some libav* include statements

Fix all include statements of the form:

   #include "libav.../..."

These come from MPlayer times, when FFmpeg was somehow part of the
MPlayer build tree, and this form was needed to prefer the local files
over system FFmpeg.

In some cases, the include statement wasn't needed or could be replaced
with mpv defined symbols.