graphics/py-pillow: Enable default support for LCMS

Convert the LCMS option, which installs graphics/lcms2 and enables the
bindings for LCMS (= Little Color Management System), into a default one.

The library liblcms2.so from graphics/lcms2 is already installed implicitly
through graphics/openjpeg using the default option JPEG2000 but Pillow's own
main library, _imaging.so, is not linked against it.

Thus do the same as in r504993 by making the LCMS option as default to get
real use out of liblcms2.so that has been already there.

Approved by: koobs (maintainer)