meson: Don't try to install dri drivers unless one is built

This confused the with_dri flag which is meant to control Direct
Rendering Infrastructure, not classic drivers

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/mesa/drivers/dri/meson.build

@@ -32,7 +32,7 @@ if with_dri_i965
   subdir('i965')
 endif
 
-if with_dri
+if dri_drivers != []
   libmesa_dri_drivers = shared_library(
     'mesa_dri_drivers',
     dummy_cpp,  # see meson #2180