package/mesa3d: always pass --with-platforms

package/mesa3d: always pass --with-platforms

If --with-platforms is not used mesa3d defaults to x11:
https://cgit.freedesktop.org/mesa/mesa/tree/configure.ac?h=17.1#n1641
https://cgit.freedesktop.org/mesa/mesa/tree/configure.ac?h=17.1#n1659

This will break configure when x11 is not needed because the defconfig
has no mesa3d drivers enabled. To solve the problem we always pass
--with-platforms, even with empty values and also for non-egl builds.

Fixes
http://autobuild.buildroot.net/results/d16/d16b39d16b5bee5c09b1e996941a275a4337c3c1/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>