configure: bail out if building svga without libdrm

configure: bail out if building svga without libdrm

With recent commit we removed the NEED_NONNULL_WINSYS checks when
selecting the hardware (inc svga) winsys. svga has only one winsys
that explicitly requires libdrm (via it's bundled version of
vmwgfx_drm.h) but configure.ac never really checks for it.

Add the check early to prevent people from shooting themselves when
they select the driver but lack libdrm.

$ ./autogen.sh --disable-dri --disable-egl --disable-gallium-llvm
--with-dri-drivers=swrast --with-gallium-drivers=svga,swrast

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=82539
Cc: "10.2 10.3" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>