linux-phytec-ti

scripts/sphinx-pre-install: cleanup Gentoo checks

On Gentoo, the portage changes for ImageMagick to work are
always suggested, even if already applied. While the two
extra commands should be harmless, add a check to avoid
reporting it without need.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>