meson: don't build gallium dri target if gallium is disabled

Otherwise -Dgallium-drivers= will cause libmesa_gallium to be built and
the megadriver install script to attempt to install drivers without any
actual drivers being built.

fixes: 66f97f6640 ("meson: build radeonsi")
Reported-by: Rafael Antognolli <rafael.antognolli@intel.com>
Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Tested-by: Rafael Antognolli <rafael.antognolli@intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>

src/gallium/meson.build

 # TODO: virgl
 # TODO: winsys/sw/xlib
 # TODO: clover
-if with_dri
+if with_dri and with_gallium
   subdir('targets/dri')
 endif
 # TODO: xlib-glx