meson: Guard freedreno build with with_gallium_freedreno.

This prevents build failures when libdrm_freedreno is unavailable,
which started happening after the ir3_compiler build was enabled.

(Patch by Rob, commit message by Ken).

Fixes: fecd04a66a ("freedreno/ir3: fix standalone compiler meson build")
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/gallium/meson.build

@@ -30,7 +30,9 @@ subdir('drivers/rbug')
 subdir('drivers/radeon')
 subdir('drivers/radeonsi')
 subdir('drivers/nouveau')
-subdir('drivers/freedreno')
+if with_gallium_freedreno
+  subdir('drivers/freedreno')
+endif
 subdir('drivers/softpipe')
 if with_gallium_vc4
   subdir('drivers/vc4')