avcodec/x86/simple_idct: Empty MMX state in ff_simple_idct_mmx

We currently mostly do not empty the MMX state in our MMX
DSP functions; instead we only do so before code that might
be using x87 code. This is a violation of the System V i386 ABI
(and maybe of other ABIs, too):
"The CPU shall be in x87 mode upon entry to a function. Therefore,
every function that uses the MMX registers is required to issue an
emms or femms instruction after using MMX registers, before returning
or calling another function." (See 2.2.1 in [1])
This patch does not intend to change all these functions to abide
by the ABI; it only does so for ff_simple_idct_mmx, as this
function can by called by external users, because it is exported
via the avdct API. Without this, the following fragment will
assert (on x86_32, as ff_simple_idct_mmx is not used on x64):
    int16_t __attribute__ ((aligned (16))) block[64];
    AVDCT *dct = avcodec_dct_alloc();
    dct->idct_algo = FF_IDCT_AUTO;
    avcodec_dct_init(dct);
    dct->idct(block);
    av_assert0_fpu();

[1]: https://raw.githubusercontent.com/wiki/hjl-tools/x86-psABI/intel386-psABI-1.1.pdf

Reviewed-by: Kieran Kunhya <kierank@obe.tv>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/x86/simple_idct.asm

@@ -845,6 +845,7 @@ INIT_MMX mmx
 
 cglobal simple_idct, 1, 2, 8, 128, block, t0
     IDCT
+    emms
 RET
 
 INIT_XMM sse2