multimedia/dav1d: convert SDL option to subpackage

[jbeich]

Test Plan

$ poudriere testport -j 132amd64 multimedia/dav1d
[...]
===========================================================================
=>> Checking shared library dependencies
 0x0000000000000001 NEEDED               Shared library: [libc.so.7]
 0x0000000000000001 NEEDED               Shared library: [libdav1d.so.7]
 0x0000000000000001 NEEDED               Shared library: [libm.so.5]
 0x0000000000000001 NEEDED               Shared library: [libthr.so.3]

$ poudriere testport -j 132amd64 multimedia/dav1d~sdl
[...]
===========================================================================
=>> Checking shared library dependencies
 0x0000000000000001 NEEDED               Shared library: [libSDL2-2.0.so.0]
 0x0000000000000001 NEEDED               Shared library: [libc.so.7]
 0x0000000000000001 NEEDED               Shared library: [libdav1d.so.7]
 0x0000000000000001 NEEDED               Shared library: [libm.so.5]