x86: huffyuvdsp: Move inline assembly to init file

This avoids a link failure with MMX disabled as now code and
initialization are compiled under the same condition.
x86: Unconditionally compile blockdsp and svq1enc init files

This avoids a link failure with MMX disabled as the init functions
are referenced unconditionally.
x86: build: Only compile FDCT code if MMX is enabled

All other files containing purely inline assembly are treated the same way.