Documentation/sphinx: make it possible to build latexdocs without pdflatex

Building latexdocs doesn't actually require $(PDFLATEX). Move the checks
for it to the pdfdocs target which does require it, and specifically
outside of the target in order to not depend on latexdocs when we can't
build pdfdocs anyway.

Reviewed-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Tested-by: Markus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>